Electronically non-adiabatic processes occur across a range of systems of interest, including photosynthetic light harvesting, fluorescent proteins, UV damage to DNA, and organic photovoltaic device excitonics. Our understanding of non-adiabatic processes can be aided greatly by ab initio excited-state dynamics simulations; however, before such simulations become routine, more work needs to be done to improve the robustness of excited-state electronic structure methods. In particular, the workhorse method of the field, state-averaged complete active space self-consistent field (SA-CASSCF), exhibits a problem arising from the use of state averaging, vital for a proper description of electronic degeneracies, that creates an imbalanced treatment of the electronic states included in the state averaging and those outside it, resulting in potential energy surface discontinuities when these states cross. We previously showed that this issue can be avoided with a dynamic weighting scheme, wherein the state average weights vary according to the energetic separation of the states. The resulting dynamically weighted complete active space self-consistent field (DW-CASSCF) method is seen to smooth out the potential energy discontinuities while retaining the overall shape of SA-CASSCF curves; however, in the initial implementation, state gradients were computationally expensive, and nonadiabatic couplings had not been derived. Using a Lagrangian approach, we have reformulated analytical gradients of DW-CASSCF to provide state gradients and non-adiabatic coupling vectors at essentially the same computational cost as these quantities from SA-CASSCF, thus enabling ab-initio excited-state dynamics simulations. We demonstrate DW-CASSCF excited-state dynamics on the UV response of the green fluorescent protein chromophore and see good agreement with recent Time-Resolved Photoelectron Spectroscopy experiments.

Reformulated analytical gradients for dynamically weighted complete active space self-consistent field

2019 Fall National Meeting

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