Performance of the Effective-characteristic-polynomial $\Pi$2 Method for Diatomic Molecules: Basis-set Dependencies and Vibrational Levels

Herbert H. H.Homeier and M. D. Neef
Email: herbert.homeier@na-net.ornl.gov
Institut für Physikalische und Theoretische Chemie
Universität Regensburg
D-93040 Regensburg, Germany

Abstract:

The performance of the recently introduced $\Pi$2 method [1] is investigated for some diatomic molecules. For this end, ground state energies are calculated at the MP4 level for various basis sets of increasing size. With negligible extra effort, the $\Pi$2, F4, and [2/2] estimators are obtained, together with information on the reliability of the basic perturbation series [1]. The results are compared to more expensive CCSD(T) results. Also, electronic energy hypersurfaces are calculated at these levels. As a further possibility to test the performance of the method, vibrational frequencies and other spectroscopic constants of diatomic molecules are calculated by fitting different analytic functions to the hypersurfaces obtained by different methods and compared to experimental data.

[1] H. H. H. Homeier, Correlation energy estimators based on Møller-Plesset perturbation theory, J. Mol. Struct. (Theochem), 366:161-171, 1996.

Category: Ab initio

Keywords: Correlation energy, Perturbation theory, Morse potential, Anharmonicity constant, Hypersurface, Spectroscopic constants