Violation of Hund's rule by spin polarization in molecules

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.