Ab initio Spin-Mapping Non-adiabatic Dynamics Simulations of Photochemistry

We perform on-the-fly non-adiabatic molecular dynamics simulations using the recently developed spin-mapping formalism. Two quantum dynamics approaches based on this mapping formalism, (i) the fully linearized spin-LSC and (ii) the partially linearized spin-PLDM, are explored using the quasi-diabatic propagation scheme. We have performed dynamics simulations in four \textit{ab initio} molecular models for which benchmark \textit{ab initio} multiple spawning data has been published. We find that the spin-LSC approach routinely outperforms the spin-PLDM approach and yields a roughly equivalent accuracy to the previously reported symmetric quasi-classical approach. We further explore the underpinnings of the spin-PLDM correlation function by decomposing its various contributions stemming from its summation over all the terms, $N^2$, in the density matrix-focused initial conditions where $N$ is the number of states in the quantum subsystem. Finally, we found an approximate form of the spin-PLDM correlation function that simplifies the simulation and reduces the computational costs from $N^2$ to $N$.