An empirical boundary potential for water droplet simulations

An empirical modified boundary potential has been derived to correct the structural perturbations arising from the presence of the vacuum boundary in the simulation of spherical TIP4P water systems. The potential is parameterized for a 12.0‐Å sphere of TIP4P water and gives improved number density and orientational sampling behavior. It is also transferable to both larger and smaller simulation systems with only a moderate degradation in performance. Free‐energy calculations have been conducted for the perturbation of a TIP4P water molecule to methane under aqueous conditions, and the modified boundary potential gives results consistent with those from simulations using periodic boundary conditions. However, simple half‐harmonic boundary potentials give unsatisfactory number density, orientational sampling, and free‐energy results. Moreover, use of the modified boundary potential results in a negligible increase in simulation time. It is envisaged that the modified boundary potential will find use in free‐energy perturbation calculations on proteins with a solvent sphere centered on the active site. © 1995 by John Wiley & Sons, Inc.