EFFECTS OF SWITCHING-FUNCTIONS ON THE BEHAVIOR OF LIQUID WATER IN MOLECULAR-DYNAMICS SIMULATIONS

The effect of truncating long-ranged interactions on a wide range of water properties is probed by comparing abrupt and switched cutoff methods to the Ewald summation method in molecular dynamics simulations at 300 K. It is found that the switching function reduces the self-diffusion coefficients by a factor of 3 relative to abrupt truncation and by a factor of 2 relative to the Ewald summation results. The switching function also makes the liquid more ordered and the intermolecular interactions stronger than either abrupt truncation or the Ewald summation method. These observed differences in water properties are interpreted in terms of a fictitious retarding force introduced by the switching function. In general, the Ewald summation results are closer to those of abrupt truncation than those obtained with switching, suggesting that the former method may be preferred when the Ewald method cannot be used. For completeness, we also characterize the commonly used water potential employed in the above studies. This model, which is used in conjuction with the consistent valence force field (CVFF), is comparable to other similar water potentials in its agreement with experiment. The intermolecular energy and equilibrium density agree particularly well and reproduce the experiment to within 3%. The largest discrepancy is observed for the self-diffusion coefficient which is predicted to be twice as large as the experimental value. However, this difference is in line with other comparable models and reflects the general inadequacy of a simple three-site Lennard-Jones plus electrostatic model, which ignores polarization and other many-body effects.