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 w ater properties is probed by comparing abrupt and switched cutoff meth ods to the Ewald summation method in molecular dynamics simulations at 300 K. It is found that the switching function reduces the self-diffu sion coefficients by a factor of 3 relative to abrupt truncation and b y a factor of 2 relative to the Ewald summation results. The switching function also makes the liquid more ordered and the intermolecular in teractions stronger than either abrupt truncation or the Ewald summati on method. These observed differences in water properties are interpre ted in terms of a fictitious retarding force introduced by the switchi ng function. In general, the Ewald summation results are closer to tho se 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 wate r potential employed in the above studies. This model, which is used i n conjuction with the consistent valence force field (CVFF), is compar able to other similar water potentials in its agreement with experimen t. The intermolecular energy and equilibrium density agree particularl y well and reproduce the experiment to within 3%. The largest discrepa ncy is observed for the self-diffusion coefficient which is predicted to be twice as large as the experimental value. However, this differen ce is in line with other comparable models and reflects the general in adequacy of a simple three-site Lennard-Jones plus electrostatic model , which ignores polarization and other many-body effects.