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Ewald summation and physically equivalent methods such as particle-meshEwald, kubic-harmonic expansions, or Lekner sums are commonly used to calculatelong-range electrostatic interactions in computer simulations of polar andcharged substances. The calculation of pressures in such systems isinvestigated. We find that the virial and thermodynamic pressures differbecause of the explicit volume dependence of the effective, resummed Ewaldpotential. The thermodynamic pressure, obtained from the volume derivative ofthe Helmholtz free energy, can be expressed easily for both ionic and rigidmolecular systems. For a system of rigid molecules, the electrostatic energyand the forces at the atom positions are required, both of which are readilyavailable in molecular dynamics codes. We then calculate the virial andthermodynamic pressures for the extended simple point charge (SPC/E) watermodel at standard conditions. We find that the thermodynamic pressure exhibitsconsiderably less system size dependence than the virial pressure. From ananalysis of the cross correlation between the virial and thermodynamicpressure, we conclude that the thermodynamic pressure should be used to drivevolume fluctuations in constant-pressure simulations.Comment: RevTeX, 19 pages, 2 EPS figures; in press: Journal of Chemical  Physics, 15-August-199

Pressure calculation in polar and charged systems using Ewald summation: Results for the extended simple point charge model of water