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Ab Initio Theory for Accurate Spectroscopic Constants and Molecular Properties

  1. David P. Tew1,
  2. Wim Klopper2,
  3. Rafał A. Bachorz2,
  4. Christof Hättig3

Published Online: 15 SEP 2011

DOI: 10.1002/9780470749593.hrs007

Handbook of High-resolution Spectroscopy

Handbook of High-resolution Spectroscopy

How to Cite

Tew, D. P., Klopper, W., Bachorz, R. A. and Hättig, C. 2011. Ab Initio Theory for Accurate Spectroscopic Constants and Molecular Properties. Handbook of High-resolution Spectroscopy. .

Author Information

  1. 1

    University of Bristol, Bristol, UK

  2. 2

    Karlsruher Institut für Technologie (KIT), Karlsruhe, Germany

  3. 3

    Ruhr-Universität Bochum, Bochum, Germany

Publication History

  1. Published Online: 15 SEP 2011

Abstract

Having access to highly accurate potential energy surfaces (PESs) is a prerequisite for the ab initio calculation of accurate spectroscopic constants and molecular properties. The construction of such surfaces, however, is a formidable task. It not only requires solving of the time-independent, nonrelativistic, clamped-nuclei, electronic Schrödinger equation of the molecular n-electron system at the highest possible level of approximation, but it also requires inclusion of small energy terms that are often neglected at other occasions. Such terms are, for example, due to nonadiabatic and relativistic effects (scalar and nonscalar). Nevertheless, the largest sources of error in calculating PESs are most often due to the inadequate description of the nonrelativistic electronic wave function of the molecular n-electron system in the Born–Oppenheimer approximation. This article is concerned with these sources of error, which are related to the truncation of the one-electron basis set of atomic functions and to the incomplete expansion of the wave function in a basis of n-electron functions. In particular, these errors diminish only extremely slowly with the extension of the one-electron basis set when the wave function is expanded in terms of n-electron functions that are (antisymmetrized) products of one-electron functions (orbitals). Coupled-cluster theory in conjunction with explicitly correlated two-electron geminals (instead of the more commonly used products of two orbitals) appears to be a promising approach toward obtaining an adequate description of the clamped-nuclei, nonrelativistic electronic wave function and, thus, for the calculation of highly accurate PESs, from which accurate spectroscopic constants and molecular properties may be obtained.

Keywords:

  • electronic-structure theory;
  • high accuracy;
  • basis set error;
  • explicit correlation;
  • perturbation theory;
  • coupled-cluster theory;
  • potential energy surfaces;
  • equilibrium geometries;
  • harmonic wavenumbers;
  • fundamental frequencies