Understanding complex materials often requires investigating multiple, tightly coupled time and length scales. Neither atomistic nor coarse-grained simulations are often able to adequately capture all the relevant scales. To combine the efficiency of coarse-grained models with the accuracy of atomistic models for systems that require atomistic resolution only locally, for example at a reactive group, mixed-resolution models have been developed. These models use a coarse-grained description for the part of the system distant from an active site and atomistic description for the active site and its direct environment. Since the active zone may diffuse during a molecular simulation, the simulation algorithm needs to permit an on-the-fly reclassication of atoms as they transition between the high- and low-resolution regimes. Recently, we derived a Hamiltonian and presented a simulation protocol for mixed-resolution systems that allows for such a change in resolution of selected groups of atoms during a molecular dynamics simulation [1]. A microcanonical simulation protocol conserves energy and angular and linear momentum. In this paper, we briefly present our novel algorithm and compare it to previous mixed resolution algorithms [2,3]. Subsequently, we present simulation results for the structure and diffusion of tetrahedral model molecules and a water molecule in hexane. These model simulations permit identifing challenges in mixed resolution simulations and how they can be overcome.

[1] Heyden, A.; Truhlar, D. G., Conservative algorithm for an adaptive change of resolution in mixed atomistic/coarse-grained multiscale simulations. Journal of Chemical Theory and Computation 2008, 4, (2), 217-221.

[2] Praprotnik, M.; Delle Site, L.; Kremer, K., Adaptive resolution molecular-dynamics simulation: Changing the degrees of freedom on the fly. Journal of Chemical Physics 2005, 123, (22), 224106.

[3] Ensing, B.; Nielsen, S. O.; Moore, P. B.; Klein, M. L.; Parrinello, M., Energy conservation in adaptive hybrid atomistic/coarse-grain molecular dynamics. Journal of Chemical Theory and Computation 2007, 3, (3), 1100-1105.