On the reaction field for interaction site models of polar systems

It is rigorously shown that the fluctuation formula, which is used in simulations to calculate the dielectric constant of interaction site models, corresponds to the reaction field with an individual site cut-off rather than with the usual molecular center of mass truncation. Within the molecular cut-off scheme, a modified reaction field is proposed. An influence of the truncation effects is discussed and examined by actual Monte Carlo simulations for a MCY water model.