Ground and excited state CASPT2 geometry optimizations of small organic molecules

Authors

  • Christopher S. Page,

    1. Dipartimento di Chimica, Università di Siena, via Aldo Moro, I-53100 Siena, Italy
    Current affiliation:
    1. Biomolecular Modelling Laboratory, Cancer Research UK, 44 Lincoln's Inn Fields, London GB-WC2A 3PX
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  • Massimo Olivucci

    Corresponding author
    1. Dipartimento di Chimica, Università di Siena, via Aldo Moro, I-53100 Siena, Italy
    2. Centro per lo Studio dei Sistemi Complessi, Università di Siena, via Tommaso Pendola 37, I-53100 Siena, Italy
    • Dipartimento di Chimica, Università di Siena, via Aldo Moro, I-53100 Siena, Italy
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Abstract

A method for computing second-order multiconfigurational perturbation theory (CASPT2) energy gradients numerically has been implemented and applied to a range of elementary organic chromophores, including 1,3 butadiene, acrolein, and two protonated Schiff bases. Geometries of ground and excited states—as well as conical intersections—are compared with the corresponding CASSCF structures, illustrating the effect of including the correction for dynamical electron correlation. It is shown that the differences between the two methods are not readily categorized, but that, while individual changes in bond lengths can be quite large (∼0.01–0.02 Å), the natures and CASPT2 energetics of the structures remain similar. Exceptions to this tend to be systems that have a strong ionic character and that are not well described at the CASSCF level. Basis set effects (double- vs. triple-ζ) were examined for a limited number of examples, and found to be quite dramatic at both levels of theory. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 298–309, 2003

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