Coarse-graining of small molecules in inhomogeneous systems through local-density dependent potentials

Abstract

We use a coarse-graining strategy that augments two-body interactions with a local-density-dependent potential to explicitly account for many-body effects. Here, we focus on free-standing films, systems with strongly varying spatial densities, which challenge coarse-grained (CG) models based on pair potentials derived from bulk simulations. We test our approach for benzene (one-site mapping) and for the larger OLED host molecule mCBP (two-site mapping), finding excellent agreement in the neighbor statistics and density profiles obtained from reference all-atom simulations. We further analyze the role of the local-density length-scale and identify an optimal range guided by structural correlations in the mapped all-atom reference. Finally, we probe temperature transferability by parameterizing the CG model at a single temperature and predicting coexistence over a wider temperature range. While the CG simulations yield stable films, quantitative deviations emerge in the vapor–liquid coexistence densities away from the fit temperature. We show that coexistence predictions are significantly improved by interpolating between CG models parameterized at selected temperatures, providing a practical route to enhanced transferability without reparameterization at every state point.