FINITE REPRESENTATION OF AN INFINITE BULK SYSTEM - SOLVENT BOUNDARY POTENTIAL FOR COMPUTER-SIMULATIONS

An approach is developed to obtain statistical properties similar to t hose of an infinite bulk system from computer simulations of a finite cluster. A rigorous theoretical formulation is given for the solvent b oundary potential which takes the influence of the surrounding bulk in to account. The solvent boundary potential is the configuration-depend ent solvation free energy of an effective cluster composed of an arbit rary solute and a finite number of explicit solvent molecules embedded inside a hard sphere of variable radius; the hard sphere does not act directly on the solute or the explicit solvent molecules, and its rad ius varies according to the instantaneous configurations. The formulat ion follows from an exact separation of the multidimensional configura tional Boltzmann integral in terms of the solvent molecules nearest to the solute and the remaining bulk solvent molecules. An approximation to the solvent boundary potential is constructed for simulations of b ulk water at constant pressure, including the influence of van der Waa ls and electrostatic interactions. The approximation is illustrated wi th calculations of the solvation free energy of a water molecule and o f sodium and potassium ions. The influence of bulk solvent on the conf ormational equilibrium of molecular solutes is illustrated by performi ng umbrella sampling calculations of n-butane and alanine dipeptide in water. The boundary potential is tested to examine the dependence of the results on the number of water molecules included explicitly in th e simulations. It is observed that bulk-like results are obtained, eve n when only the waters in the first hydration shell are included expli citly.