Cut-off radius effect of the isotropic periodic sum method for polar molecules in a bulk water system

Molecular dynamics simulation has been applied for water to compare the isotropic periodic sum (IPS) method for polar molecules (IPSp) to the normal IPS (IPSn) method and the Ewald sum by evaluating the diffusion coefficient and liquid structure. In our previous study, we have applied the IPSn method for bulk water and found notable deviation of the radial distribution function g(r). In this work, the IPSp gives a good estimation for the potential energy and the self-diffusion coefficient at a cut-off radius, r _c, greater than 2.2nm while avoiding the notable deviation of g(r) which appeared in the case of IPSn. The distance-dependent Kirkwood factor G _k(r) was also calculated, and the truncation of a long-range interaction of the cut-off-like method (such as cut-off with or without the switch function and the reaction field) shows serious shortcomings for dipole-dipole correlations in bulk water systems. This was observed by comparing the shape to that of the Ewald sum. G _k(r) of the cut-off-like method greatly deviates from that of the Ewald sum. However, the discrepancy of G _k(r) for IPSp method was found to be much less than that of other typical cut-off-like methods. We conclude that the IPSp method is an adequately accurate technique for estimating transport coefficients and the liquid structure of water in a homogeneous system at long cut-off distances.