PRESSURE CALCULATION IN POLAR AND CHARGED SYSTEMS USING EWALD SUMMATION - RESULTS FOR THE EXTENDED SIMPLE POINT-CHARGE MODEL OF WATER

Ewald summation and physically equivalent methods such as particle-mes h Ewald, kubic-harmonic expansions, or Lekner sums are commonly used t o calculate long-range electrostatic interactions in computer simulati ons of polar and charged substances. The calculation of pressures in s uch systems is investigated. We find that the virial and thermodynamic pressures differ because of the explicit volume dependence of the eff ective, resummed Ewald potential. The thermodynamic pressure, obtained from the volume derivative of the Helmholtz free energy, can be expre ssed easily for both ionic and rigid molecular systems. For. a system of rigid molecules, the electrostatic energy and the forces at the ato m positions are required, both of which are readily available in molec ular dynamics codes. We then calculate the virial and thermodynamic pr essures for the extended simple point charge (SPC/E) water model at st andard conditions. We find that the thermodynamic pressure exhibits co nsiderably less system size dependence than the virial pressure. From an analysis of the cross correlation between the virial and thermodyna mic pressure, we conclude that the thermodynamic pressure should be us ed to drive volume fluctuations in constant-pressure simulations. (C) 1998 American Institute of Physics.