Free energy and entropy of a dipolar liquid by computer simulations

Understanding dynamics in coarse-grained models. I. Universal excess entropy scaling relationship

Coarse-grained (CG) models facilitate an efficient exploration of complex systems by reducing the unnecessary degrees of freedom of the fine-grained (FG) system while recapitulating major structural correlations. Unlike structural properties, assessing dynamic properties in CG modeling is often unfeasible due to the accelerated dynamics of the CG models, which allows for more efficient structural sampling. Therefore, the ultimate goal of the present series of articles is to establish a better correspondence between the FG and CG dynamics. To assess and compare dynamical properties in the FG and the corresponding CG models, we utilize the excess entropy scaling relationship. For Paper I of this series, we provide evidence that the FG and the corresponding CG counterpart follow the same universal scaling relationship. By carefully reviewing and examining the literature, we develop a new theory to calculate excess entropies for the FG and CG systems while accounting for entropy representability. We demonstrate that the excess entropy scaling idea can be readily applied to liquid water and methanol systems at both the FG and CG resolutions. For both liquids, we reveal that the scaling exponents remain unchanged from the coarse-graining process, indicating that the scaling behavior is universal for the same underlying molecular systems. Combining this finding with the concept of mapping entropy in CG models, we show that the missing entropy plays an important role in accelerating the CG dynamics.

Thermodynamic properties of a molecular dipolar liquid using the two-phase thermodynamic approach

A modified version of the original two-phase thermodynamic approach has been extended to evaluate the thermodynamic properties of molecular systems. Its basic assumption states that the density of states can be decomposed into solid-like and gas-like components. The solid part has been approximated by that of a set of harmonic oscillators, whereas a subset composed of rough hard spheres has been considered for the gas part. In this new approach, molecules have been modelled as rotating hard spheres that experience elastic collisions. The technique has been tested on a system made up of dipolar diatomic molecules, and it leads to very good results for total entropy, potential energy reference and heat capacity. Translation and rotation solid components of the overall spectra have been compared to the real part of the instantaneous normal mode vibrational densities of states. Similarities between them reinforce the validity of the two-phase thermodynamic approach.