BOUNDARY-CONDITION EFFECTS IN SIMULATIONS OF WATER CONFINED BETWEEN PLANAR WALLS

In computer simulations of water between hydrophobic walls the results exhibit a strong dependence upon the boundary conditions applied. Wit h the minimum image (MI) convention the water molecules tend to be ori entationally ordered throughout the simulation cell (Valleau, J. P., a nd Gardner, A. A., 1987, J. chem. Phys., 86, 4162) whereas, if a spher ical cut-off (SC) is applied, strong orientational order is found only in the immediate vicinity of the surface (Lee, C. Y., McCammon, J. A. , and Rossky, P. J., 1984, J. chem. Phys., 80, 4448). These conflictin g observations have remained unresolved, and clearly raise troubling q uestions concerning the validity of simulation results for water betwe en surfaces of all types. In the present paper we explore this problem by carrying out a detailed analysis of the results obtained with vari ous types of boundary condition. These include Ewald calculations carr ied out with a central simulation cell adapted to describe the slab ge ometry of interest. It is shown that the order observed in MI calculat ions is an artefact of that particular truncation. The reason for this is isolated and discussed. Similar problems are found if a cylindrica l cut-off is employed. The Ewald and SC methods gave qualitatively sim ilar results for systems similar to those considered in previous simul ations. However, for some geometries problems can also arise with the SC method. We conclude that in general the slab-adapted Ewald method i s the safest choice.