Spin‐orbit coupling of DFT/MRCI wavefunctions: Method, test calculations, and application to thiophene

Abstract

During the past decade the one‐center mean‐field approximation has proven to be a very appropriate framework for the accurate description of spin‐orbit effects at the correlated all‐electron level. Here, a new efficient code, SPOCK, is introduced that calculates spin‐orbit matrix elements in the one‐center mean‐field approximation for multireference CI wave functions. For the first time, the computation of spin‐dependent interactions within a Kohn‐Sham orbital based CI (DFT/MRCI) scheme 1 is made possible. The latter approach is suitable for large scale systems with up to 100–200 valence electrons. Test calculations are performed on well‐known diatomic molecules and the thiocarbonyl pyranthione. Spin‐orbit matrix elements show good agreement with their Hartree‐Fock orbital based counterparts but are obtained at considerably lower expense, thus demonstrating the power of the method. As an application singlet‐triplet couplings in thiophene are investigated that are important for the photophysics and photochemistry. Spin‐orbit matrix elements between all π → π* excited states are found to be small. Considerably larger spin‐orbit matrix elements are observed only for cases in which π → σ* excited configurations are involved. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 824–833, 2002