title: 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) scheme1 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 pi --> pi* excited states are found to be small. Considerably larger spin-orbit matrix elements are observed only for cases in which pi --> sigma* excited configurations are involved.
authors.affiliation: GMD Research Center for Information Technology, Scientific Computing and Algorithms Institute (SCAI), Schloss Birlinghoven, 53754 St. Augustin, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Kleinschmidt
authors.forename: Jörg
authors.initials: J
authors.lastname: Tatchen
authors.forename: Christel M
authors.initials: CM
authors.lastname: Marian