EFFECT OF ELECTROSTATIC FORCE TRUNCATION ON INTERFACIAL AND TRANSPORT-PROPERTIES OF WATER

The importance of accurately accounting for all Coulombic forces in mo lecular dynamics simulations of water at interfaces is demonstrated by comparing the Ewald summation technique with various spherical trunca tion methods, The increased structure induced by truncation methods at 12 Angstrom leads to water/vapor surface tensions and surface potenti als that are respectively 50% and 100% greater than obtained with Ewal d. The orientational polarization of water at the lipid/water interfac e is analyzed within the Marcelja-Radic theory of the hydration force, yielding decay parameters of 2.6 and 1.8 Angstrom for spherical trunc ation and Ewald, respectively, as compared with 1.7-2.1 Angstrom obtai ned from experiment. Bulk water transport properties such as the visco sity and diffusion constants differ by as much as 100% between simulat ions carried out with and without truncation; this may be related to o rdering in the neighborhood of the cutoff radius. The diffusion consta nt calculated from the Ewald simulation is significantly further from experiment than the cutoff result, pointing out the need to reparametr ize the TIP3P water model for use with Ewald summation. Appendices des cribe a method for carrying out the Ewald summation on a distributed m emory parallel computer and other computational details relevant when simulating large systems.