Dynamics at intersystem crossings is fundamental to many processes in chemistry and biology. Ab Initio Multiple Spawning (AIMS) method was originally developed to describe dynamics at conical intersections where derivative coupling is responsible for nonadiabatic transitions between electronic states with the same spin multiplicities. We extended the applicability of the AIMS method to intersystem crossings where transitions between electronic states of different spin multiplicities are mediated by relativistic spin-orbit coupling. In AIMS method nuclei are described by the basis set of frozen multidimensional Gaussians propagating on electronic potential energy surfaces calculated on-the-fly. For electronic structure calculations we used GAMESS suite of programs. The new methodology was used to describe the intersystem crossings between 3B1 and 1A1 states of SiH2, GeH2. In the test calculations potential energies and gradients were obtained at CASSCF(6,6)/6-31G(d) level of theory. Spin-orbit coupling matrix elements were calculated with CASCI method using Breit-Pauli Hamiltonian with full 1-electron and partial 2-electron contributions. The electronic state lifetimes and intersystem crossing rate constants were estimated using the first-order kinetics equation for a reversible unimolecular reaction. The obtained rate constants were compared with the results predicted by time-independent nonadiabatic transition state theory with transition probabilities calculated using Landau-Zener and Delos formulas.

Ab initio multiple spawning method for intersystem crossings: Spin-forbidden transitions between 3B1 and 1A1 states of SiH2, GeH2

251st National Meeting (2016)

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