BREAKDOWN OF LINEAR RESPONSE FOR SOLVATION DYNAMICS IN METHANOL

We present the results of a molecular dynamics (MD) simulation study of solvation dynamics in methanol. Using nonequilibrium MD, we have calculated the relaxation rate of the solute-solvent potential energy change due to an instantaneous dipole creation in a diatomic solute, a model for solvent response to a charge-transfer electronic transition of a probe molecule. Linear response (LR) approximations to this process were obtained from equilibrium MD. LR results for methanol in the presence of the ground- and excited-state solutes differ substantially from each other and from nonequilibrium solvation dynamics. We show that O-H bond motions dominate early solvation dynamics, but that other relaxation mechanisms subsequently become important, and explain why LR breaks down much more seriously for methanol than for other polar solvents previously studied by MD.