Liquid water simulation: a critical examination of cutoff length.

Cutoff treatment is the simplest approach for evaluating intermolecular interactions in molecular dynamics simulations. It has been believed that increasing cutoff length makes simulation results better. On the contrary, our results of the bulk water simulations studied within the range of cutoff lengths, 9-18 A, showed an opposite tendency: the artifact was enhanced by increasing the cutoff length. Especially, in terms of the distance dependent Kirkwood factor GK(r), it was clearly shown that the orientational behavior of water molecules becomes gradually worse as the cutoff length becomes longer. The artifact enhanced by the increased cutoff length led to a reported spurious artifact, i.e., phase transition [Y. Yonetani, Chem. Phys. Lett. 406, 49 (2005)]. Though the cutoff artifact was largely reduced by adopting a force switching technique, it did not completely remove the anomalous cutoff length dependence of the artifact. These results suggest that increasing the cutoff should not be attempted regardless of whether the switching force is adopted or not.