Multiscale Coarse-Graining of Polarizable Models through Force-Matched Dipole Fluctuations.

Abstract

An extension of the multiscale coarse-graining method (MS-CG) to polarizable coarse-grain (CG) models is presented. In the extension, force matching is used to derive charged dimers that mimic the dipole behavior, including electronic polarizability, of fine resolution systems. The extended MS-CG method separates short-range and electrostatic forces and treats the polarization interactions by representing dipole fluctuations with a harmonic bond reminiscent of the Drude oscillator. The new method is first tested on several flexible alcohols, where the transferable head groups in the atomistic field lead to transferable electrostatics in the CG field. The method is then applied to a polarizable methanol model, where the CG dimer is able to match the atomistic dipole distribution. Force fields are benchmarked with radial distribution functions and dielectric constants. The static dielectric constants agree well with atomistic models, but the faster dynamics in the CG ensembles leads to significant deviations in the frequency-dependent permittivity. Model transferability is checked by measuring response to static external fields through a dipole coupling order parameter. Effective polarizabilities are inferred from the dimer harmonic bonds, and an ad hoc correction is used to improve one CG force field's dipole magnitude response to electric fields. Broader applicability is explored through a small set of amines, and several multibead models are tested where CG dipole orientations are restricted through intramolecular forces, capturing constructive and destructive dipole interactions within single molecules.