This work presents exchange potentials for specific orbitals calculated by inverting Hartree–Fock wavefunctions. This was achieved by using a Depurated Inversion Method. The basic idea of the method relies on the substitution of Hartree–Fock orbitals and eigenvalues into the Kohn–Sham equation. Through inversion, the corresponding effective potentials were obtained. Further treatment of the inverted potential should be carried on. The depuration is a careful optimization which eliminates the poles and also ensures the fullfilment of the appropriate boundary conditions. The procedure developed here is not restricted to the ground state or to a nodeless orbital and is applicable to all kinds of atoms. As an example, exchange potentials for noble gases and term-dependent orbitals of the lower configuration of Nitrogen are calculated. The method allows to reproduce the input energies and wavefunctions with a remarkable degree of accuracy.

This work presents exchange potentials for specific orbitals calculated by inverting Hartree–Fock wavefunctions. The characteristic poles and divergences caused by the presence of orbital nodes (formal and spurious) are dealt with the depurated inversion method. The method allows to obtain accurate singularity-free effective one-electron local potentials with only few parameters.

Temozolomide was paired with guanine, 6-selenoguanine, and 6-thioguanine, as well as the SH tautomer of the latter. The potential energy surface of each heterodimer was searched for all minima, using Dispersion-Corrected Density Functional Theory and MP2 methods. Among the dozens of minima, three categories were observed. Stacked geometries place the aromatic systems of the two molecules parallel to one another, while the two systems are roughly perpendicular to one another in a second category. Also found are coplanar structures held together by H-bonds. Dispersion proves to be a dominating attractive force for the stacked structures, less so for perpendicular, and smallest for the coplanar dimers. Geometries and energetics are relatively insensitive to S and Se substitution, but tautomerization reverses relative stabilities of different geometries.

The temozolomide molecule (TMZ) is attracting growing attention by virtue of its ability to serve as a DNA alkylating agent. First-principles calculations provide insight into the interaction of TMZ with guanine analogues. Replacement of O of guanine by S or Se yields three different geometries of heterodimer complexes with TMZ. This substitution has relatively little effect on preferred structures or energetics. Most minima contain H-bonds, even those that are dominated by π-π interactions.

Zinc oxide (ZnO) nanostructures have attracted much interest due to their potential applications in various fields including optoelectronics, glass industries, and solar cells. These compounds hold the promise of creating new materials that can advance energy technologies. In this work, a series of (ZnO)_{6} clusters with selenium and tellurium applied as substitutional impurities has been studied. The investigated structures have been produced through the doping of (ZnO)_{6} clusters by replacing an oxygen atom with a selenium or a tellurium atom at each time. The ground state geometric parameters of (ZnO)_{6} structures, containing selenium or tellurium atoms as substitutional impurities, were calculated using density functional theory (DFT) with B3LYP and LanL2DZ basis set. Excited state energies and absorption wavelengths were computed using time-dependent-DFT (TDDFT). For the calculation of emission wavelengths, Hartree–Fock configuration interaction singles (HF/CIS) has been used in order to perform the excited state geometry optimization. This work led to some important results that can be helpful for developing novel THz sensitive materials and imaging detectors that may be an alternative to x-rays detectors for radiology as well as for the development of solar cells and electroluminescent diodes. Zinc oxide (ZnO) nanostructures have attracted growing interest due to their potential applications in many technological fields, including optoelectronics, the glass industry, and energy. The presence of impurities, in particular selenium and tellurium, in ZnO-based clusters can affect their structural and spectroscopic properties. Some of these doped nanostructures have favorable Terahertz emission characteristics that make them good candidates for applications in biology and medicine.

Zinc oxide (ZnO) nanostructures have attracted growing interest due to their potential applications in many technological fields, including optoelectronics, the glass industry, and energy. The presence of impurities, in particular selenium and tellurium, in ZnO based clusters can affect their structural and spectroscopic properties. Some of these doped nanostructures have favorable Terahertz emission characteristics that make them good candidates for applications in biology and medicine.

Metal dithiolene complexes have been extensively used for the elaboration of conducting or magnetic materials, whose molecular arrangements can be sensitively influenced by strong and directional noncovalent interactions. In this work, halogen bonds between *N*-methyl-3-halopyridinium cations and cyano-substituted anion radicals, [M(C_{2}S_{2})_{2}CN]^{−•} (M = Ni, Pd, and Pt), were systematically investigated at the M06 level of theory. The CN···X interactions in these systems are predicted to be considerably strong and play a vital role in the controlling of the crystal structures of molecular conductors and magnets. The electrostatics contributes mainly to the attraction of these halogen bonds, while the orbital interaction is also important. Particularly, the formation of these interactions has little effect on the distribution of SOMOs, which is strongly delocalized on the two dithiolene moieties. These results will assist in the design of functional materials that exhibit exotic conducting or magnetic properties.

Halogen bonding interactions between *N*-methyl-3-halopyridinium cations and cyano-substituted anion radicals were systematically studied using density functional theory calculations. These interactions are predicted to be essentially linear and very strong. Therefore, they should play a pivotal role in influencing the crystal structures of molecular conductors or magnets. In addition, the electrostatics plays a major role in the attraction of these interactions.

Polycyclic aromatic hydrocarbon (PAH) molecules are responsible for a family of features, the aromatic infrared bands, which are observed in many astronomical environments. The photophysics and photochemistry of PAHs in space is strongly influenced by anharmonicity, which has seldom been considered in astrophysical modeling because of the lack of available data or because too computationally demanding. Recent developments of second-order vibrational perturbation theory applied to density functional theory now allow the computation of the anharmonic quartic force field anharmonic spectra of medium-large molecules like PAHs in a time-efficient way. It is now possible to have a better grasp of the effects of anharmonicity on the vibrational spectrum of PAHs also thanks to the comparison with gas-phase, high-resolution experimental data. In this perspective, the quality of these anharmonic spectra for PAH molecules is reviewed and discussed in view of a robust astrochemical model for PAH molecules.

Polycyclic aromatic hydrocarbon (PAH) molecules are of great interest for astronomers as they the responsible of the aromatic infrared bands, found in many astrophysical environments. A detailed knowledge of PAH anharmonicity will permit to use them as probes of physical and chemical conditions in space. This perspective takes a look to the theoretical advance in the DFT anharmonic vibrational spectra for PAHs and discuss their potential and the current challenges.

A detailed analysis on the effect of spherical impenetrable confinement on the structural properties of two-electron ions in
states has been performed. The energy values of 1*sns* [
] (
) states of helium-like ions (
) are estimated within the framework of Ritz variational method using explicitly correlated Hylleraas-type basis sets. The correlated wave functions used here are consistent with the finite boundary conditions due to spherical confinement. A comparative study between the singlet and triplet states originating from a particular electronic configuration shows incidental degeneracy and the subsequent level-crossing phenomena. The thermodynamic pressure felt by the ion inside the sphere pushes the energy levels toward continuum. Critical pressures for the transition to strong confinement regime (where the singly excited two-electron energy levels cross the corresponding one-electron threshold) as well as for the complete destabilization are also estimated.

The structural modifications of helium-like ions are investigated within the framework of Ritz variational method by using explicitly correlated Hylleraas-type basis sets consistent with the finite boundary conditions due to spherical confinement. A comparative study between the singlet and triplet states originating from a particular electronic configuration shows incidental degeneracy and the subsequent level-crossing phenomena. Critical radius and pressures corresponding to destabilization of the ions are also discussed.

Astrochemistry is an interdisciplinary field involving chemistry, physics, and astronomy, which encompasses astronomical observations, modeling, as well as theoretical and experimental laboratory investigations. In the frame of the latter, this contribution provides an overview on the computational approaches supporting and complementing rotational spectroscopy experiments applied to astrochemical studies. The focus is on the computational strategies that permit accurate computations of structural and rotational parameters as well as of energetics and on their application to case studies, with particular emphasis on the so-called “astronomical complex” organic molecules.

Molecular species in the gas-phase are usually detected via their rotational signatures. These are accurately obtained in laboratory spectroscopy studies that are increasingly assisted by quantum-chemical calculations to guide and support the spectral recording and analysis. The support of quantum chemistry to rotational spectroscopy in the field of astrochemistry is the topic covered by this perspective. An overview of state-of-the-art quantum-chemical techniques for the evaluation of rotational parameters has been given.

The Fenna–Matthews–Olson (FMO) complex—a pigment protein complex involved in photosynthesis in green sulfur bacteria—is remarkably efficient in transferring excitation energy from light harvesting antenna molecules to a reaction center. Recent experimental and theoretical studies suggest that quantum coherence and entanglement may play a role in this excitation energy transfer (EET). We examine whether bipartite quantum nonlocality, a property that expresses a stronger-than-entanglement form of correlation, exists between different pairs of chromophores in the FMO complex when modeling the EET by the hierarchically coupled equations of motion method. We compare the results for nonlocality with the amount of bipartite entanglement in the system. In particular, we analyze in what way these correlation properties are affected by different initial conditions. It is found that bipartite nonlocality only exists when the initial conditions are chosen in an unphysiological manner and probably is absent when considering the EET in the FMO complex in its natural habitat. It is also seen that nonlocality and entanglement behave quite differently in this system. In particular, for localized initial states, nonlocality only exists on a very short time scale and then drops to zero in an abrupt manner. As already known from previous studies, quantum entanglement between chromophore pairs, on the other hand, is oscillating and exponentially decaying and follow thereby a pattern more similar to the chromophore population dynamics. The abrupt disappearance of nonlocality in the presence of nonvanishing entanglement is a phenomenon we call *nonlocality sudden death*; a striking manifestation of the difference between these two types of correlations in quantum systems.

Quantum effects during excitation energy transfer (EET) between chromophores in the Fenna–Matthews–Olson photosynthetic complexes have attracted considerable attention recently. Whether quantum nonlocality, a stronger-than-entanglement form of correlation between chromophore pairs, can exist in the EET is examined. For states initially localized at a single chromophore, nonlocality exists on a short time scale and then drops to zero in an abrupt manner. Nonlocality is vanishing when modeling the system in its natural habitat.

A detailed theoretical description of metal–ligand interactions in the case of the simple isoelectronic transition metal series and one ligand is presented. This task is performed in terms of the local and nonlocal topology-based formalisms of the electronic density and its decomposition into paired and unpaired contributions. The analysis is mainly focused on the nature of the carbon–metal interactions under the traditional chemical back-donation phenomena and the relationship with the existence of two-electron three-center (2e-3c) complex patterns of bonding, that is, 2e-3c atomic interactions. For these simple prototypical systems, which seem to be adequate examples to describe the topologic features of such electron distribution in terms of the density point of view, both phenomena, that is, the back-donation and the 2e-3c interactions, are mutually exclusive.

Topological tools, as, for example, maps of the Laplacian function, are very useful instruments to visualize the concentration of the electron density. These allow describing the interactions in the electron distributions undergoing back-donation phenomena and its relationships with 2e-3c type nonconventional patterns of bonding. The details of density deformation due to the interactions between atoms can be extracted by this approach for any type of bonding situations.

We illustrate the state of the art in time-dependent calculations on systems of chemical interest. In particular, our exposition covers the Gaussian multiconfiguration time-dependent Hartree/variational multiconfiguration Gaussian approach in nuclear dynamics, where the scope is that of explaining dynamical effects in various physicochemical processes. Conversely, the Rice–Ramsperger–Kassel–Marcus/master equation kinetic methods are also examined, used to calculate rate constants of gas phase processes (used in the modeling of combustion, atmospheric, and astrochemical processes).

To study properties of a system as prepared in specific initial conditions, time-dependent calculations are typically necessary. Quasiclassical or quantum dynamical calculations provide information of energy distributions among degrees of freedom and their consequences in terms of physicochemical effects. For statistically averaged evolution, one enters the realm of kinetics. Time-dependent methods are compared and contrasted, in particular Gaussian-based nuclear dynamics to kinetics in the microcanonical and macrocanonical ensemble.

We present the theory of semilocal exchange-correlation (XC) energy functionals which depend on the Kohn–Sham kinetic energy density (KED), including the relevant class of meta-generalized gradient approximation (meta-GGA) functionals. Thanks to the KED ingredient, meta-GGA functionals can satisfy different exact constraints for XC energy and can be made one-electron self-correlation free. This leads to a better accuracy over a wider range of properties with respect to GGAs, often reaching the accuracy of hybrid functionals, but at much reduced computational cost. An extensive survey of the relevant literature on existing KED dependent XC functionals is provided, considering nonempirical, semi-empirical, and fully empirical ones. A deeper analysis and a wide benchmark are presented for functionals derived considering only exact constraints and parameters obtained from model and/or atomic systems.

Kinetic energy density (KED) dependent semilocal exchange-correlation functionals are attracting growing interest due to their advantageous accuracy/computational cost ratio compared to that of hybrid functionals. KED dependent functionals can be subdivided in five classes based on their additional dependence on the density gradient and/or the density Laplacian. In particular, nonempirical KED dependent meta-generalized gradient approximation (K-meta-GGA) functionals are constructed only on exact constraints and parameters from model/atomic systems.

Aim of this contribution is to review some recent quantum mechanical approaches used to compute the redox potentials of transition metal complexes, with the emphasis on copper and iron species, which are particularly relevant in inorganic biochemistry and in synthetic chemistry of bio-mimetic compounds. The paper presents also new DFT results obtained on Cu and Fe aquo ions in the framework of the Thermodynamic Integration and Grand Canonical Ensemble approaches. Such results show that without explicit inclusion of water molecules in the external solvation shells (even using a continuum solvation model) also very advanced methodologies fail to predict the redox potential in an acceptable manner. This is a confirmation of some previous studies which however never addressed this specific problem along the aforementioned approaches. Better results are obtained, on the contrary, on a series of Cu(II) complexes with Gly, Ala, en, Im, and water ligands with coordination type
or N_{4}. In this case, the complexes are surrounded by nine water molecules which may partially alleviate the inadequacy of the continuum solvent models, especially in the case of highly positively charged species.

Calculations of the redox potential of copper complexes, in particular in aqueous solution, are difficult due to the significant structural rearrangements that often accompany the reduction from Cu(II) to Cu(I) complexes and the specific interactions between water molecules and metal complexes. MD-based approaches, such as thermodynamic integration and the grand canonical ensemble method with explicit solvent representation, partly alleviate the inadequacies of methods based on single calculations of the species using implicit solvent models.

Density functional theory calculations were carried out to investigate the Diels–Alder cycloaddition between cyclopentadiene and C_{60} after the encapsulation of Li^{+} ion with transition states identified and confirmed by intrinsic reaction coordinate calculations. Our results showed that the Li^{+}-encapsulation results in a lower energy barrier and the presence of counter anion
can further reduce the energy barrier, making the trend in agreement with the experimental results. In addition, the influencing factors on the reactivity of Li^{+}-encapsulated fullerenes such as counter anion and the position of Li^{+} in C_{60} were discussed. This study aims to provide a better understanding of Diels–Alder reaction with Endohedral Metallofullerenes to allow more efficient functionalization of fullerenes.

While encapsulated metal ions are known to affect the rate of Diels–Alder reactions catalyzed by fullerenes, counter anions are not considered to be a significant factor in the reaction. First-principles calculations reveal that the counter anion selection, for example, in this study, can further enhance the rate of Diels–Alder reaction by attracting the encapsulated cation to a position closer to fullerene surface and thus facilitating more charge transfer.

We present a theoretical study of Feshbach resonances in HCO, using an extension of a previous projection theory [Y. Wang and J. M. Bowman, J. Chem. Phys. 139, 154303 (2013)] that makes use of projections of the normal modes of HCO onto a one-dimensional rectilinear path along the imaginary-frequency normal mode of the dissociation saddle point. HCO dissociation is strongly mode specific, because the CH-stretch is nearly coincident with this path. The projection theory predicts that HCO dissociates with a subpicosecond lifetime for the CH-stretch excited to the second overtone ( ), in agreement with rigorous calculations done 20 years ago. However, the CO-stretch and HCO bend have small projections on the path and so no dissociation is predicted from the simple theory, even for highly excited states. By making an extension of the projection theory to describe coupling of these modes to the CH-stretch, dissociation rates can be obtained for these modes. Semi-quantitative results are obtained using vibrational self-consistent field/virtual-state configuration interaction calculations with the code MULTIMODE. The dissociation lifetimes for many states involving excitation of these modes are compared with previous rigorous calculations and experiment and encouraging agreement is found.

A theoretical study of Feshbach resonances in HCO is presented using an extension of projection theory, which makes use of projections of the normal modes of HCO onto a one-dimensional path along the imaginary-frequency normal mode of the dissociation saddle point. The approach is extended to calculate the dissociation rate for the modes with small or zero projections by considering the coupling of these modes.

Considering different solar dyes configuration, four novel metal-free organic dyes based on phenoxazine as electron donor, thiophene and cyanovinylene linkers as the -conjugation bridge and cyanoacrylic acid as electron acceptor were designed to optimize open circuit voltage and short circuit current parameters and theoretically inspected. Density functional theory and time-dependent density functional theory calculations were used to study frontier molecular orbital energy states of the dyes and their optical absorption spectra. The results indicated that D2-4 dyes can be suitable candidates as sensitizers for application in dye sensitized solar cells and among these three dyes, D3 showed a broader and more bathochromically shifted absorption band compared to the others. The dye also showed the highest molar extinction coefficient. This work suggests optimizing the configuration of metal-free organic dyes based on simple D- -A configuration containing alkyl chain as substitution, starburst conformation, and symmetric double D- -A chains would produce good photovoltaic properties.

Considering different solar dyes configuration, four novel metal-free organic dyes are designed and theoretically investigated. Phenoxazine, thiophene and cyanovinylene linkers, and cyanoacrylic acid act as electron donor, -conjugation bridge, and electron acceptor, respectively. Density functional theory and time-dependent density functional theory calculations provide deeper insight into the geometrical and electronic properties of sensitizers.

Recent photoemission spectroscopic (X-ray photoemission spectra) study revealed less dramatic chemical changes for pyrimidine (PyM, 1, 3-diazine) with in its ionization potential. Present systematic study using density functional theory calculations shows that PyM is indeed quite different from its diazine isomers (PyD, 1, 2-diazine and PyA, 1, 4-diazine). It is discovered that the most stable isomer PyM is relaxed from C_{2V} to C_{1} point symmetry with a total electronic energy deduction of −15.86 kcal.mol^{−1}. Although not substantial, PyM has the smallest molecule shape (electronic spatial extent) and the largest HOMO-LUMO energy gap of 5.65 eV; only one absorption band in the region of 200–300 nm of the UV-Vis spectrum but three clusters of chemical shift in the carbon and hydrogen NMR spectra. The energy decomposition analyses revealed that the interaction energy (Δ*E*_{Int}) of PyM is preferred over PyA by 4.08 kcal.mol^{−1} and over PyD by 22.32 kcal.mol^{−1}, with the preferred NCN bond revealed by graph theory.

The question why nature chooses pyrimidine as a core component of DNA bases over other diazine isomers, that is, pyrazine and pyridazine, requires an investigation at the molecular level. In addition to being the most stable isomer, pyrimidine possesses unique properties including a single absorption band in the region of 200–300nm UV-Vis spectrum and s-like HOMO. Pyrimidine is also the preferred structure in the energy decomposition analyses with the favorite NCN bond revealed by graph theory.

In light of a recent comment by Vladimir Mandelshtam, we provide a brief, theoretically motivated, justification of our methodology. We show that our error metric provides a valid lower bound for the parameter estimation error through both exact calculations and numerical examples. Additionally, we prove the validity of Parseval?s identity.

The electron delocalization range function EDR(
(Janesko et al., *J. Chem. Phys*. 2014, 141, 144104) quantifies the width of the one-particle density matrix about point
, measuring aspects of delocalization. Here, we explore the EDR in stretched and compressed chemical bonds. The EDR illustrates how compressing chemical bonds localizes the one-particle density matrix about the reference point, and captures aspects of fractional occupancy, and left-right correlation in stretched covalent bonds.

Electron delocalization is fundamental to chemical bonding. Electron delocalization range function EDR (
) quantifies the degree to which electrons at point
in a calculated wavefunction delocalize over distance *d*. The EDR illustrates electron localization in compressed bonds as well as delocalization, fractional occupancy, and left-right correlation in stretched bonds. The EDR also shows how simple mean-field theories over-delocalize stretched bonds, a delocalization error that is fixed in accurate multireference calculations.

In this tutorial review, we provide a comprehensive description of a multiscale methodology tailored to the calculation of nuclear magnetic resonance (NMR) relaxation data of flexible molecules, based on the definition, parametrization, and solution of a Smoluchowski equation defined for a set of relevant molecular coordinates. While the method is applicable in principle to any collection of internal degrees of freedom, here we focus on flexibility described in terms of torsion angles under the paradigm of what we call the diffusive chain model. The theoretical basis of the multiscale stochastic approach to NMR spectroscopy is provided in detail. Computational aspects are discussed, pointing at a suggested set of specific software packages. To give to this contribution a hands-on component, a self-contained tutorial is made available as Supporting Information with the discussion of some examples taken from recently published studies.

This tutorial review provides a comprehensive description of a multiscale integrated computational approach to the calculation of nuclear magnetic resonance relaxation data of flexible molecules in solution based on the definition, *ab initio* parametrization, and solution of a stochastic diffusive equation for a set of relevant molecular coordinates.

We investigate the open-shell systems of the methyl radical, the allyl radical and molecular oxygen using Monte Carlo configuration interaction. We look at whether modifying Monte Carlo configuration interaction to use the increased flexibility of unrestricted Hartree-Fock orbitals offers any benefits in the description of these systems by the resulting compact wavefunctions. The expectation of the total spin squared when using Slater determinants is calculated to investigate if pure spin states can be evolved by Monte Carlo configuration interaction and how this might be accelerated. We consider the multireference character of these open-shell systems. To this end we demonstrate how a previously introduced way [J. P. Coe and M. J. Paterson, J. Chem. Theory Comput. 11, 4189 (2015)] of viewing a multireference indicator of P.-O. Löwdin [P.-O. Löwdin, Phys. Rev. 97, 1474 (1955)] in terms of the spatial entanglement can be generalized to open-shell systems.

Open-shell systems may present challenges for electronic structure methods. Monte Carlo configuration interaction (MCCI) is investigated on such systems. This method is adapted to use unrestricted Hartree-Fock orbitals and calculate expectations of the total spin squared. Pure spin states are seen to be approached in the evolution of the MCCI wavefunction despite starting with substantial spin contamination. A way to view a multireference indicator in terms of the spatial entanglement is generalized to open-shell systems.

Albeit its chemical inertness, rare gas doping can substantially enhance the quantum size effect of nanocrystals, yet little attention has been paid on this fascination and the mechanism behind remains unclear. Here, we show that the rare gas dopant breaks bonds of its neighboring atoms, which effects the same to atomic under-coordination on the bond strain, energy quantum entrapment, and valence electron polarization of Li and Na clusters. Consistency between density functional theory calculation and the bond-order-length-strength correlation prediction revealed that the bond strain by 16.86% and 21.12% before and after He doping for Na_{30} clusters. Observations suggest an effective yet simple means to modulate the physical properties by doping the inert gases.

Known for their chemical inertness, rare gas doping can substantially enhance quantum size effect of nanocrystals. Calculations show that rare gas doping of Li and Na clusters can be used to tailor bond energy, bond energy density, and atomic cohesive energy by breaking the bonds of their neighboring atoms. In Na_{30} clusters bond strain increases substantially after He doping.

A quantum-classical transition equation in complex space is derived in the framework of the complex quantum Hamilton–Jacobi formalism. The transition equation is obtained by subtracting a complex-valued quantum potential term from the complex-extended time-dependent Schrödinger equation (TDSE). It is shown that the nonlinear transition equation is equivalent to a linear scaled TDSE with a rescaled Planck's constant. Employing the quantum momentum function defined by the gradient of the complex action, we can analyze the quantum-classical transition of physical systems using complex transition trajectories. Complex transition trajectories are presented for the free Gaussian wave packet, the harmonic oscillator, and the Morse potential. This study demonstrates that the transition equation provides a continuous description for the quantum-classical transition of physical systems in complex space.

A nonlinear quantum-classical transition equation is derived in complex space. This equation is equivalent to a linear scaled time-dependent Schrodinger equation, and it provides a continuous description of the quantum-classical transition for physical systems in complex space. The complex transition trajectory formalism is developed for the transition equation and complex transition trajectories are analyzed for the free Gaussian wave packet, the harmonic oscillator, and the Morse potential. This study provides an alternative way to study the quantum-classical correspondence.

The physical properties of gas phase metal monoacetylide (MCCH, where M = Sc–Zn) molecules have been evaluated with coupled cluster theory and multireference configuration interaction. Metal monoacetylides are isovalent to well-studied metal monocyanides (MCN) and metal monoisocyanides (MNC), which suggests the MCCH series will have similar electronic structure. Some of the MCCH molecules, such as ScCCH, FeCCH, and CoCCH share a trait with their isoelectronic MCN/MNC counterparts: profound multireference character and low-lying excited electronic states. Also like the MCN/MNC molecules, the MCCH species have rather large ligand dissociation energies. Due to the prevalence of C_{n}H polyyne molecules discovered in the interstellar medium, members of the MCCH series likely exist as relevant astrochemicals and could be involved in prebiotic reactions.

The physical properties of gas phase metal monoacetylide (MCCH, where M = Sc–Zn) molecules have been evaluated with coupled cluster theory and multireference configuration interaction. Some of the MCCH molecules share a trait with their isoelectronic metal monocyanide counterparts: profound multireference character and low-lying excited electronic states. Members of the MCCH series likely exist as relevant astrochemicals and could be involved in prebiotic reactions.

In this work, we present an overview on how density functional theory calculations can be used to design novel electrocatalytic materials for fuel cells. In particular, we focus the attention on non-metal doped graphene systems, which were reported to present excellent performances as electrocatalysts for the oxygen reduction reaction (ORR) at the cathodic electrode of fuel cells and are, thus, considered promising substitutes of platinum or platinum alloys electrodes. The methodology, originally proposed by Nørskov et al. (J. Phys. Chem. B 2004, 108, 17886) for electrochemical processes at metal electrodes, is revisited and applied specifically to doped graphene. Finite molecular models of graphene are found to perform as well as periodic models for localized properties or reactions. Therefore, the sophisticated molecular quantum mechanics methodologies can be safely used to compute reliable Gibbs free energies of reaction in an aqueous environment for the various steps of reduction (at the cathode) or of oxidation (at the anode). Details of the reaction mechanisms and accurate cell onset- or over-potentials can be derived from the Gibbs free energy diagrams. The latter are computational quantities which can be directly compared to experimentally obtained cell overpotentials. Modeling electrocatalysis at fuel cells is, thus, an extremely powerful and convenient tool to improve our understanding of how fuel cells work and to design novel potentially active electrocatalytic materials. In this work, we present two specific applications of B-doped graphene, as electrocatalysts for the ORR at a half-cell cathode and for the methanol oxidation reaction (MOR) at a half-cell anode.

Non-metal doped graphene has great potential to replace platinum as electrode material in energy-related devices. Quantum chemical simulation of a prototypical fuel cell is a powerful tool to understand the electrochemical processes at the cathode (oxygen reduction reaction) and at the anode (methanol oxidation reaction) and to design novel and efficient electrocatalytic materials.

The role of Virtual Reality (VR) tools in molecular sciences is analyzed in this contribution through the presentation of the Caffeine software to the quantum chemistry community. Caffeine, developed at Scuola Normale Superiore, is specifically tailored for molecular representation and data visualization with VR systems, such as VR theaters and helmets. Usefulness and advantages that can be gained by exploiting VR are here reported, considering few examples specifically selected to illustrate different level of theory and molecular representation.

The massive evolution of three dimensional interactive computer graphics and Immersive Virtual Reality (IVR) technologies offers nowadays interesting opportunities in molecular representation. This contribution presents a new molecular viewer, called Caffeine, specifically designed to exploit the latest IVR technologies, highlighting its applications in the visualization and analysis of data coming from QM and MM computations.

The theoretical modeling of chemical reactions in condensed phase still represents an open field of investigation for theoretical–computational chemistry. In this context, we have proposed a methodology (the Perturbed Matrix Method [PMM]), based on the first principles of statistical mechanics and on the use of molecular simulations, which has demonstrated its ability in quantitatively predicting the kinetics and the thermodynamics of chemical reactions in complex atomistic environments. In this study, we demonstrate the features of PMM by modeling the resonant Electron Transfer bimolecular reaction between ferrocene and ferrocenium in solution. The obtained results, despite the adopted approximations and the intrinsic limitations of the approach, are in very good agreement with the experimental data and demonstrate the ability of the method for addressing complex reactions in solution and for evaluating the kinetics of slow events out of the potentialities of the state-of-the-art molecular simulations.

A theoretical–computational approach, based on molecular dynamics simulations and Perturbed Matrix Method, is proposed for modeling bimolecular electron transfer reactions in condensed phase. The resonant electron transfer bimolecular reaction between ferrocene and ferrocenium ion in acetonitrile solution is utilized as case study. The agreement with the experimental data demonstrates good performance of the proposed theoretical–computational procedure despite several approximations and drawbacks commented in the study.

The use of variational nuclear motion programs to compute line lists of transition frequencies and intensities is now a standard procedure. The ExoMol project has used this technique to generate line lists for studies of hot bodies such as the atmospheres of exoplanets and cool stars. The resulting line list can be huge: many contain 10 billion or more transitions. This software update considers changes made to our programs during the course of the project to allow for such calculations. This update considers three programs: Duo which computed vibronic spectra for diatomics, DVR3D which computes rotation-vibration spectra for triatomics, and TROVE which computes rotation-vibration spectra for general polyatomic systems. Important updates in functionality include the calculation of quasibound (resonance) states and Landé g-factors by Duo and the calculation of resonance states by DVR3D. Significant algorithmic improvements are reported for both DVR3D and TROVE. All three programs are publically available from ccpforge.cse.rl.ac.uk. Future developments are also considered.

Molecular spectra provide important remote sensing fingerprints. However, hot molecules can undergoing very large numbers of possible transitions: billions for even fairly small molecules such as methane. Nuclear motion software based on the use of the variational principle used to compute line lists is discussed and the adaptation of the programs to the demands of computing huge lists of molecular transitions described.

In this review, the origins of astrochemistry and the initial applications of quantum chemistry to the discovery of new molecules in space are discussed. Furthermore, more recent successes and failures of quantum astrochemistry are explored. Finally, the application of quantum chemistry to the chemical study of space is driving developments in large-scale computational science. Consequently, cloud computing and large molecule computations are discussed. Astrochemistry is a natural application of quantum chemistry. The ability to analyze routinely and completely the structural, spectroscopic, and electronic properties of any given molecule, regardless of its laboratory stability, make this tool a necessary component for astrochemical analysis. The sizes of the computations scaling with the number of electrons and degrees-of-freedom can become limiting, but proper choices of methods can provide unique insights. The chemistry of the Earth is a small snapshot of the chemistries available in the universe at large, and the flexibility inherent within computation make quantum chemistry an excellent driver of new knowledge in fundamental molecular science as well as in astrophysics. © 2016 Wiley Periodicals, Inc.

Quantum chemistry has long been a necessary tool in the elucidation of astronomical spectroscopy. This review highlights the past, present, and future of what quantum chemistry has to offer the astrochemist.

On page 1575, Matteo Bonfanti and Rocco Martinazzo examine reactions at surfaces under a magnifying glass. Fruitful combination of theory, modelling, and simulations provides a powerful tool to understand the dynamics of atoms and molecules at the gas-solid interface. This is pictured on the cover with a simple illustration of two prototypical recombination processes, the Eley-Rideal (left) and the Langmuir-Hinshelwood (right) reactions. Image credit goes to Matteo Bonfanti. (DOI: 10.1002/qua.25192)

Sr_{2}Fe_{1.5}Mo_{0.5}O_{6−δ} (SFMO) is a promising electrode material for solid oxide electrochemical cells. This perspective highlights the role of first-principles investigations in unveiling SFMO structural, electronic, and defect properties. In particular, DFT + U provides a reliable and convenient tool for extensive studies on strongly correlated transition-metal oxides, as SFMO and related systems. The SFMO excellent performances are ascribed to a mixed oxide ion-electron conductor character. Crucial features are the easy formation of oxygen vacancies and the low oxide migration barrier heights. Aliovalent doping with K^{+} enables convenient hydration and effective proton transport in bulk SFMO. This opens a route toward new promising triple-conductor oxides. Besides discussion of specific SFMO applications, our results help to uncover general perovskite-oxide features and new design principles for oxide- and proton-conducting solid oxide fuel cell electrodes. © 2016 Wiley Periodicals, Inc.

Quantum-chemical investigations can boost the development of advanced materials for energy conversion technologies. In the context of solid oxide fuel cells, the case of Sr_{2}Fe_{1.5}Mo_{0.5}O_{6−δ}-based electrodes exemplifies the successful application of DFT methods to the rational design of triple-conductor oxides, highlighting the key structure–property–function relationships that determine the oxide and proton bulk transport processes in perovskite oxides.

Computational modeling methods play a prominent role in the field of homogeneous catalysis given that reaction cycles tend to be multistep complicated processes, where the active catalytic species or intermediates are challenging or impractical to study via experimental approaches. At the same time, as such processes are purely molecular, modeling is viable even if very often computationally costly. The possibility to equally well access stable experimentally observable catalytic intermediates and short-life inaccessible species, such as high energy intermediates and, especially, transition states allows exploring and unraveling complete reaction mechanisms. The quantum mechanical description of intricate catalytic cycles is discussed here for three homogeneous catalytic systems all focused on the use of hydrogen as a potential zero-emission energy carrier for the future.

Computational modeling methods play a prominent role in the field of homogeneous catalysis given that reaction cycles tend to be multistep complicated processes, where the active catalytic species or intermediates are challenging or impractical to study via experimental approaches. The quantum mechanical description of intricate catalytic cycles is discussed here for three homogeneous catalytic systems all focused on the use of hydrogen as a potential zero-emission energy carrier for the future.

Molecular structure is one of the most relevant concepts in chemistry. It plays a central role in determining molecular and spectroscopic properties: a mandatory prerequisite for a thorough understanding of the chemical and physical properties of molecules is in fact represented by the knowledge of their geometrical structures. While in some fields a qualitative description of the molecular structure might be sufficient, in many others, like for example spectroscopy, a quantitative, and accurate determination is mandatory. Nowadays, the most advanced computational methodologies allow reliable structural predictions able to fulfil the proper accuracy requirements. This contribution provides an overview on this topic, focusing on the computational strategies that permit accurate equilibrium structure determinations for systems ranging from small molecules to medium-sized building-blocks of biomolecules.

The development of computational strategies that permit accurate equilibrium structure determinations for systems ranging from small molecules to medium-sized biosystems is one of the main challenges in theoretical chemistry. Composite approaches have been developed in order to account for both electron correlation and basis-set effects at the best possible level. The additivity approximation is at the heart of these approaches: the various contributions are evaluated separately at the highest possible level and then combined together.

Solid-state NMR spectroscopy and computational approaches such as Molecular Dynamics (MD) simulations and Density Functional Theory have proven to be very useful and versatile techniques for studying the structure and the dynamics of noncrystalline materials if a direct comparison between experiment and theory is established. In this review, the basic concepts in first-principle modeling of solid-state NMR spectra of oxide glasses are presented. There are three theoretical ingredients in the computational recipe. First, classical or *ab initio* molecular dynamics simulations are employed to generate the structural models of the glasses of interest. Second, periodic Density Functional Theory calculations coupled with the gauge including projector augmented-wave (GIPAW) algorithm form the basis for the *ab initio* calculations of NMR parameters (chemical shielding and quadrupolar parameters). Finally, Spin-effective Hamiltonian are employed to simulate the solid-state NMR spectra directly comparable with the experimental counterparts. As an example of this methodology, the investigation of the local and medium range structure of Na-Ca silicate and aluminosilicate glasses that are usually employed as simplified models for basaltic, andesitic and rhyolitic magmas will be reported. We will show how the direct comparison of the theoretical NMR spectra of MD derived structural models with the experimental counterparts allows gaining new insights into the atomistic structure of very complex oxide glasses. © 2016 Wiley Periodicals, Inc.

Molecular dynamics simulations coupled with DFT-GIPAW NMR calculations and spin-effective Hamiltonians provide a clear view of the local and medium range structure of multicomponent alumina silicate glasses.

Solvent effects on chiroptical properties and spectroscopies can be huge, and affect not only the absolute value but the sign of molecular chiroptical responses. Therefore, the definition of reliable theoretical models and computational protocols to calculate chiroptical responses and assist the assignment of the chiral absolute configuration cannot overlook the effects of the surrounding environment. Continuum solvation methodologies are successful in case of weakly interacting solute–solvent couples, whereas in case of strongly interacting systems, such as those dominated by explicit hydrogen bonding interaction, a change of strategy is required to gain a reliable modeling. In this review, a recently developed integrated Quantum-Mechanical/Polarizable molecular mechanics (MM)/polarizable continuum model (PCM) method is discussed, which combines a fluctuating charge approach to the MM polarization with the PCM. Its theoretical fundamentals, and issues related to the calculation of chiroptical responses are summarized, and the application to few representative test cases in aqueous solution is discussed.

Solvent effects have great influence on chiroptical properties of molecular systems, and therefore cannot be ignored in theoretical models. Recently developed, integrated QM/polarizable MM/Continuum strategies based on fluctuating charges have demonstrated to be powerful approaches to quantitatively reproduce chiroptical properties and spectroscopies of aqueous solutions dominated by specific hydrogen-bonding effects.

In this tutorial review, we present some effective methodologies available for the simulation of vibrational and vibrationally resolved electronic spectra of medium-to-large molecules. They have been integrated into a unified platform and extended to support a wide range of spectroscopies. The resulting tool is particularly useful in assisting the extensive characterization of molecules, often achieved by combining multiple types of measurements. A correct assessment of the reliability of theoretical calculations is a necessary prelude to the interpretation of their results. For this reason, the key concepts of the underlying theories will be first presented and then illustrated through the study of thiophene and its smallest oligomer, bithiophene. While doing so, a complete computational protocol will be detailed, with emphasis on the strengths and potential shortcomings of the models employed here. Guidelines are also provided for performing similar studies on different molecular systems, with comments on the more common pitfalls and ways to overcome them. Finally, extensions to other cases, like chiral spectroscopies or mixtures, are also discussed.

Cost-effective methodologies to simulate and interpret accurate vibrational and electronic spectra, directly comparable to experiment, are described from a theoretical and practical perspectives. Key aspects regarding their application on real cases are discussed through the design of a complete computational protocol starting from the definition of the best suited electronic structure calculation method, using thiophene and bithiophene as test cases. Possible shortcomings and strategies to overcome the limitations of those methods are also presented.

Elementary processes involving atomic and molecular species at surfaces are reviewed. The emphasis is on simple classical and quantum models that help to single out unifying dynamical themes and to identify the basic physical mechanisms that underlie the rich variety of phenomena of surface chemistry. Starting from an elementary description of the energy transfer between a gas-phase species and a surface—for both classical and quantum lattices—the key processes establishing the formation of an adsorbed phase (sticking, diffusion and vibrational relaxation) are discussed. This is instrumental for introducing the simplest chemical transformations involving adsorbed species and/or scattering of gas-phase molecules: Langmuir–Hinshelwood, Hot-Atom, and Eley–Rideal reactions forming complex molecules from elementary constituents, and dissociative chemisorption of molecules into smaller fragments. Applications are also provided illustrating the ideas developed along the way at work in real-world gas-surface problems.

Simple classical and quantum analytical models provide valuable insight into elementary dynamical processes occurring at the gas-surface interface. In particular, information can be obtained from calculations on energy transfer processes that determine the establishment of an adsorbed phase on the surface, as well as the efficiency and the energy partitioning of most gas-surface reactions.

We describe the implementation and application of a recently developed time-dependent density-functional theory (TDDFT) algorithm based on the complex dynamical polarizability to calculate the photoabsorption spectrum of large metal clusters, with specific attention to the field of molecular plasmonics. The linear response TDDFT equations are solved in the space of the density fitting functions, so the problem is recast as an inhomogeneous system of linear equations whose resolution needs a numerical effort comparable to that of a SCF procedure. The construction of the matrix representation of the dielectric susceptibility is very efficient and is based on the discretization of the excitation energy, so such matrix is easily obtained at each photon energy value as a linear combination of constant matrix and energy-dependent coefficients. The code is interfaced to the Amsterdam Density Functional (ADF) program and is fully parallelized with standard message passing interface. Finally, an illustrative application of the method to the photoabsorption of the Au_{144}(SH)_{60} cluster is presented. © 2016 Wiley Periodicals, Inc.

Efficient and accurate prediction of photoabsorption spectra for large systems is a fundamental goal of modern computational research. This is usually achieved within the time-dependent density-functional theory (TDDFT) approach but high associated computational costs limit its applicability to large systems. A recently developed TDDFT algorithm based on the complex dynamical polarizability to calculate the optical photoabsorption of large metal clusters is particularly suitable for applications in the field of plasmonics.