The non-separability of "dielectric" and "mechanical" friction in molecular systems: A simulation study

Simulations of the time-dependent friction controlling rotational, translat ional, and vibrational motions of dipolar diatomic solutes in acetonitrile and methanol have been used to examine the nature of "dielectric" friction. The way in which electrical interactions increase the friction beyond that present in nonpolar systems is found to be rather different than what is a nticipated by most theories of dielectric friction. Long-range electrostati c forces do not simply add an independent contribution to the friction due to short-ranged or "mechanical" sources (modeled here in terms of Lennard-J ones forces). Rather, the electrical and Lennard-Jones contributions are fo und to be strongly anticorrelated and not separable in any useful way. For some purposes, the mechanism by which electrical interactions increase fric tion is better viewed as a static electrostriction effect: electrical force s cause a subtle increase in atomic density in the solute's first solvation shell, which increases the amplitude of the force fluctuations derived fro m the Lennard-Jones interactions, i.e., the mechanical friction. However, e lectrical interactions also modify the dynamics of the friction, typically adding a long-time tail, which significantly increases the integral frictio n. Both of these effects must be included in a correct description of frict ion in the presence of polar interactions. (C) 2000 American Institute of P hysics. [S0021-9606(00)51112-X].