(469c) Hybrid Molecular-Continuum Simulations Using Smoothed Dissipative Particle Dynamics

A broad range of mesoscale hydrodynamic problems are characterized by multiple time and length scales, motivating "hybrid" simulation techniques that involve multiple spatial simulation domains of differing resolution. Such multiscale simulation approaches typically decouple length scales by employing a detailed description (e.g. molecular dynamics) for domains with processes occurring at the molecular level of detail, while coarse-graining bulk regions far away. For many problems, the coarse regions may be treated using the continuum approximation, with some intermediate overlap or buffer region between the atomistic and continuum domains that ensures quantities such as mass and momentum are appropriately conserved and transferred between the two domains.

We discuss a novel hybrid simulation approach based on a continuum technique called “smoothed dissipative particle dynamics” (SDPD), which is a stochastic particle method for solving the fluctuating Navier-Stokes equations. SDPD is ideally suited for coupling to molecular dynamics because it is particle-based and offers a characteristic length scale that can be tuned between molecular and continuum regimes. Our approach makes it possible to embed an atomically-resolved molecular dynamics (MD) domain within an SDPD fluid, or within a hierarchy of SDPD fluids, each at a different resolution. Because SDPD is a particle-based continuum solver, it is easily reconciled with MD since it is only necessary to mediate particle-particle interactions. The coupling is achieved by introducing a “buffer” region separating the atomistic and continuum domains, where forces are continuously interpolated using a switching function. This mixture of forces results in density deviations within the buffer which can be removed by introducing a so-called "thermodynamic force", which we reformulate into a pairwise interaction using the SDPD approach. We demonstrate that these techniques are successful in reproducing both equilibrium and non-equilibrium scenarios for simple fluids, such as the start up of shear flow. The results from these tests are in agreement with analytical theory and independent of the placement of the buffer region within the domain. We also discuss the generalization of this multiscale methodology to multicomponent systems and more complicated domain geometries and modeling setups.