A space-time coarse-grained model for particle simulations of soft matter

We have devised a simple soft-attractive-and-repulsive pair-potential model, which has a hardness parameter (,6) and a smooth cut-off, to adopt coarse-graining procedures for soft-matter. Namely, (i) Grouping for a monomer or segment of molecules; (ii) Packing for a group of molecules; and (iii) Time-averaging to take account of effective potential, U-eff(r), over a time-span. The resulting models are capable of reproducing liquid-vapor coexistence curves of real fluids at various space-time coarse-grained levels. A model with beta=3.0 is analytically representing a time-averaged U-eff() over a long time-span, gives excellent energy conservation with a very long time-step. Combining the new model with dissipative particle dynamics (DPD) method should allows us to retain the phase behavior and dynamics properties of underlying molecular systems. In this work, we have performed DPD simulations of surface-induced nano-pattern of block copolymer film, and found that the simulated surface structures resembled those patterns observed in experiment of symmetric PS-b-PMMA thin films on a PS-coated substrate.