SCF and CI Studies of Hund's rules for the effects of electronic correlation and delocalization. I

Document Type

Article

Publication Date

1-1-1983

Department

Department of Chemistry; Department of Chemical Engineering

Abstract

In connection with the reinterpretation of Hund's multiplicity rules for molecules, a detailed study has been made of the energy differences in the total energy and its components for the triplet and singlet Πu states of the hydrogen molecule and the analogous states of the four‐ and six‐membered hydrogen atom rings. For the hydrogen molecule, both SCF and CI studies indicated that the outer electron is considerably more contracted in the triplet than in the singlet state. In both approximations, the energy difference is dominated for all bond distances of chemical and physical significance by the electron‐nuclear attraction component and not by the electron repulsion component as predicted by simple first‐order perturbation theory. Although the correlation energy for each of the states is of the same magnitude as the energy differences considered here, the difference of the correlation energies is much smaller. It had little effect on the qualitative differences between these states of the hydrogen molecule. For the four‐ and six‐membered rings, SCF studies were made on the lowest singlet and triplet states where one electron was promoted from the σg to a Πu orbital. Even though the coupled electrons were more delocalized in these cases, the electron repulsion became relatively more important. However in all cases, the lower state had the highest electron repulsion energy and lower electron‐nuclear attraction. The triplet state continued to have the more contracted outer open‐shell orbital.

Publisher's Statement

Copyright © 1983 John Wiley & Sons, Inc. Publisher’s version of record: https://doi.org/10.1002/qua.560240606

Publication Title

International Journal of Quantum Chemistry

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