Electronic spin-spin coupling (SSC)
Electronic SSC is a two-electron property and hence its quantum chemical evaluation requires substantial computational resources.(Marian-2001) Combined with the notion that SSC typically contributes little to the fine-structure splitting compared to the (usually) much larger spin-orbit coupling, only few quantum chemical programs have been developed for that purpose, among them Spock.Sistr (Gilka-2008) for MRCI wave functions. SSC is interesting, however, in cases where direct spin-orbit interaction is symmetry-forbidden. For example, SSC causes first-order zero-field splitting (ZFS) of triplet-excited hydrocarbons, which significantly exceeds the second-order spin-orbit effect. A resolution-of-the-identity (RI) approximation for the four-index two-electron SSC integrals in the ORCA program (Ganyushin-2010) to which the Spock.Sistr and DFT/MRCI codes have been interfaced, eases substantially on the computational demands. Nevertheless, the investigation of 5(T...T) ↔ 1(T...T) spin interconversion, postulated to occur between quintet- and singlet-coupled triplet pairs in triplet-triplet annihilation upconversion (TTA-UC) and singlet fission (SF) of polyacene dimers, is currently out of reach. The existing serial Spock.Sistr code needs parallelization and streamlining for that purpose, as the resource (RAM, disk storage and CPU time) requirements of the current program version are outrageous.
Early-stage researchers who want to join our team working on this project should contact Prof. Christel Marian or Dr. Martin Kleinschmidt.