Evaluation of net interactions for liquid methane based on coarse-grained simulation

Abstract

The procedure for generating the coarse-grained potential based on the information of liquid state simulations is adopted for liquid methane. The spherically projected potential (SPP) approach consistently generates the effective pair potential for liquid methane, similar to previous liquid water research. The SPP procedure includes a specific feature to extract the net interactions between molecules. For liquid methane, the naïve attractive interaction is incorporated into the SPP, and the potential profile is transferred to the nearly repulsive interaction as a hard-sphere system. A comparison study using two energy profiles, the SPP and the direct interaction contribution of the SPP, gives a similar liquid structure, and the difference between these profiles reflects the existence of the structure-invariant energy profile. The features of the profiles are important for recognizing the characteristics of coarse-grained potential. In the simulation, the potential of mean force (PMF) can be generally evaluated based on the calculated liquid structure. The difference between the PMF and SPP leads to fluctuations in local number density (LND), and it expresses the many-body contribution to liquid structure on the SPP surface. The specific characteristic of the liquid water structure at an ambient condition is explained from the viewpoint of coarse-grained potential by comparing the relation between the SPP and fluctuations in LND for liquid methane with those for liquid water.