Nonadiabatic Surcafe Hopping Simulation Of Channelrhodopsin 2

Nonadiabatic QM/MM dynamics were performed using the fewest-switches surface hopping (FSSH) algorithm to investigate the early photochemical relaxation of the protonated retinal Schiff base (rPSB) in ChR2. Initial conditions were sampled from geometries extracted from ground-state Born-Oppenheimer molecular dynamics (BOMD) trajectories and subsequently promoted vertically to the second excited state (S2). The nonadiabatic trajectories were propagated at the state-averaged CASSCF (SA-CASSCF) level of theory—comprising three electronic states—to ensure a consistent description of the coupled excited-state potential energy surfaces governing the ultrafast relaxation dynamics of the chromophore. Furthermore, the isomerization pathway connecting the all-trans and 13-cis configurations was characterized by computing a relaxed scan along the torsional coordinate of the reactive double bond at the S1 state. Calculations were restricted to Chain A of the wild-type system. The QM region consisted exclusively of the retinal chromophore, capped with a methyl group. All simulations were conducted in an NVE-like ensemble utilizing the OpenMolcas/Tinker interface.