Conditional Reversible Work Coarse-Grained Models of Molecular Liquids with Coulomb Electrostatics - A Proof of Concept Study on Weakly Polar Organic Molecules

Scale bridging simulations of soft matter rely on the availability of transferable coarse-grained models. In systematic coarse-graining approaches, core principles of statistical mechanics are used to relate the coarse-grained models to the underlying molecular interactions. The conditional reversible work (CRW) method provides effective, nonbonded pair potentials by means of computing coupling free energies between mapped chemical groups. This method has so far been used almost exclusively for systems composed of apolar organic molecules, but additional challenges arise when developing coarse-grained models for polar molecules in which (long-range) electrostatic interactions are important. Herein, we present a modified formulation of the CRW method where we divide the effective interaction potential into van der Waals and electrostatic components. The shape of the effective electrostatic interaction justifies modeling the electrostatics using a Coulomb potential with point charges on each site that are equal to the net charge of the underlying group of atoms. We perform CRW calculations using two polar molecules as test cases (an ether (1,2-dimethoxyethane) and an ester (ethyl propionate)). The results of subsequent liquid state simulations indicate that the coarse-grained models obtained by the new method are of similar quality with respect to representability and thermodynamic transferability as formerly developed models for apolar systems.