Abstract
It is shown that spin polarization effects can lead to violations of Hund's multiplicity rule, particularly for singlet-triplet pairs. In systems with more than one unpaired electron we observe a dynamic spin polarization which depends on the relative spin orientation of the unpaired electrons and is different in open-shell singlet and triplet states. These effects are described by including singly substituted configurations in CI-type wavefunctions for the two states. In analyzing the contributions of the various singly substituted configurations a quantitative understanding of violations of Hund's rule is obtained; at the same time it is possible to calculate quantitatively spin polarization contributions to the correlation energies of open-shell singlet and triplet states.
A series of model calculations is performed on systems like square planar H4 and C4H4, nπ* states of formaldehyde, rotated cumulenes, NH and O2, in order to investigate how strongly spin polarization influences singlet-triplet energy splittings and what are the properties of a molecule that lead to a violation of Hund's multiplicity rule.
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Kollmar, H., Staemmler, V. Violation of Hund's rule by spin polarization in molecules. Theoret. Chim. Acta 48, 223–239 (1978). https://doi.org/10.1007/BF00549021
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DOI: https://doi.org/10.1007/BF00549021