Non-Adiabatic Excited-State Molecular Dynamics for Open-Shell Systems.

Non-adiabatic Molecular Dynamics (NAMD) of excited states has been widely used in the simulation of photoinduced phenomena. However, the inability to treat bond breaking and forming processes with single-reference electronic structure methods limits their application in photochemistry for extended molecular systems. In this work, the extension of excited state NAMD for open-shell systems is developed and implemented in the NEXMD software. We present the spin-unrestricted CIS and TD-SCF formalism for the ground and excited-states, analytical derivatives and non-adiabatic derivative couplings for the respective potential energy surfaces. This methodology is employed to study the photochemical reaction of three model molecules. The results demonstrate the advantage of the open-shell approach in modeling photochemical reactions, especially involving bond breaking processes. We find that the open-shell method lowers the reaction barrier at the bond-breaking limits resulting in larger calculated photochemical quantum yields compared to the respective closed-shell results. We also address problems related to spin-contamination in the open-shell method, especially when molecular geometries are far from equilibrium.