Spin-allowed photochemical reactions of molecular systems are typically treated by means of non-adiabatic dynamics simulations, which involve a large number of ground and excited electronic state calculations. The chosen electronic structure method must be able to treat static correlation properly and be efficient enough to sample hundreds of trajectories—a particular problem for larger systems. In this work, we investigate novel combinations of semiempirical electronic structure and active space excited state methods and their potential use in ab initio multiple spawning (AIMS)[1] simulations. The extended tight-binding schemes GFN-xTB[2] and GFN2-xTB[3] are among the most efficient and accurate semiempirical methods for electronic ground state calculations. For a better description of excited states, we augment these methods with Mulliken-approximated Fock exchange. The additional computation time due to the exchange becomes negligible if performed on graphics processing units (GPUs, see Figure 1). The combination of the modified xTB methods with active space methods, e.g., spin-restricted ensemble-referenced Kohn-Sham (REKS)[4] theory, then provides a very inexpensive electronic structure model that can be used in AIMS simulations.

Towards non-adiabatic dynamics simulations using semiempirical extended tight-binding (xTB) methods

2019 Fall National Meeting

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