Transferable potentials for phase equilibria - Coarse-Grain description for linear alkanes

Coarse-grain potentials allow one to extend molecular simulations to length and time scales beyond those accesssible to atomistic representations of the interacting system. Since the coarse-grain potentials remove a large number of interaction sites and, hence, a large number of degrees of freedom, it is generally assumed that coarse-grain potentials are not transferable to different systems or state points (temperature and pressure). Here we apply lessons learned from the parametrization of transferable atomistic potentials to develop a systematic procedure for the parametrization of transferable coarse-grain potentials. In particular, we apply an iterative Boltzmann optimization for the determination of the bonded interactions for coarse-grain beads belonging to the same molecule and separated by one or two coarse-grain bonds and parametrize the nonbonded interactions by fitting to the vapor - liquid coexistence curves computed for selected molecules represented by the TraPPE - UA (transferable potentials for phase equilibria - united atom) force field. This approach is tested here for linear alkanes where parameters for C₃H₇ end segments and for C₃H₆ middle segments of the TraPPE - CG (transferable potentials for phase equilibria - coarse grain) force field are determined and it is shown that these parameters yield quite accurate vapor - liquid equilibria for neat n-hexane to n-triacontane and for the binary mixture of n-hexane and n-hexatriacontane. In addition, the position of the first peak in various radial distribution functions and the coordination number for the first solvation shell are well reproduced by the TraPPE - CG force field, but the first peaks are too high and narrow.