Mixed Resolution Modeling of Interactions in Condensed-Phase Systems

A new mixed resolution method is developed for modeling molecular interactions that employs a distance-dependent coupling of atomistic and coarse-grained force fields. In the mixed resolution interaction (MRI) method, detailed atomistic structure is maintained over the whole system. However, the atomistic force field is used for close interparticle separations (called the atomistic zone), while at large separations the coarse-grained forces are unfolded into atomistic interactions in a way that reduces the cost of the simulation compared to standard long-range approximations or cutoff schemes. Several variations of the unfolding scheme are described. The method is applied to develop MRI models of bulk TIP3P water, based on one-site multiscale coarse-grained (MS-CG) water potentials located at the molecular centers of either mass or geometry. With a sufficiently large atomistic zone (>0.7 nm), the MRI models provide excellent simulations of the bulk water phase. MRI modeling is further illustrated for liquid methanol with both one- and two-site coarse graining. The MRI water models are then used to simulate aqueous solutions, where the solutes are treated at the atomistic level. It is shown that the MRI treatment significantly alters solute association dynamics if it relies on the MS-CG force fields obtained solely from the bulk phase. Possible modifications of the MRI procedure to improve the transferability of water potentials to heterogeneous systems are, therefore, discussed. The best result is obtained if water molecules within a preselected cutoff distance from the solute are described using only atomistic potentials. As a final example, the MRI method is applied to model a solvated phospholipid bilayer.