Recent progress in adaptive multiscale molecular dynamics simulations of soft matter

Understanding mesoscopic phenomena in terms of the fundamental motions of atoms and electrons poses a severe challenge for molecular simulation. This challenge is being met by multiscale modeling techniques that aim to bridge between the microscopic and mesoscopic time and length scales. In such techniques different levels of theory are combined to describe a system at a number of scales or resolutions. Here we review recent advancements in adaptive hybrid simulations, in which the different levels are used in separate spatial domains and matter can diffuse from one region to another with an accompanying resolution change. We discuss what it means to simulate such a system, and how to enact the resolution changes. We show how to construct efficient adaptive hybrid quantum mechanics/molecular mechanics (QM/MM) and atomistic/coarse grain (AA/CG) molecular dynamics methods that use an intermediate healing region to smoothly couple the regions together. This coupling is formulated to use only the native forces inherent to each region. The total energy is conserved through the use of auxiliary bookkeeping terms. Error control, and the choice of time step and healing region width, is obtained by careful analysis of the energy flow between the different representations. We emphasize the CG → AA reverse mapping problem and show how this problem is resolved through the use of rigid atomistic fragments located within each CG particle whose orientation is preconditioned for a possible resolution change through a rotational dynamics scheme. These advancements are shown to enable the adaptive hybrid multiscale molecular dynamics simulation of macromolecular soft matter systems.