A coarse-grained potential for liquid water by spherically projected hydrogen-bonding interactions

Abstract

A procedure for generating a coarse-grained spherically symmetric potential in order to reproduce the structure of liquid water is developed noniteratively through an all-atom Monte Carlo simulation. This procedure for determining the spherically averaged potential (SAP) extracts the effective potential of the molecular pairs from the simulation. The SAP procedure is then modified by projecting the hydrogen-bonding interactions during the all-atom simulation onto the spherically symmetric interaction. The projection suppresses the excessively attractive interaction at small intermolecular distances and allows one to derive the spherically projected potential (SPP). Although the rotation of the all-atom water molecule is accompanied by large energy fluctuations, this is not the case for the spherically symmetric potential. Given the decrease in the number of energy fluctuations in the case of coarse-graining, we scale both the SAP and the SPP. The scaled potentials for the SAP and SPP are referred to as the SAPw and SPPw, respectively. While the SAPw reproduces the liquid structure qualitatively, the SPPw reproduces the liquid structure well compared to the SAPw. Furthermore, the SAP procedure can be applied to any system involving interactions between spheres. By successively using the SAP procedure for the initial SPPw, we show that the obtained coarse-grained potentials produce a liquid structure that is similar to the initial one. Hence, this investigation provides novel insights into the characteristics of coarse-grained potentials.