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

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.