Efficient and realistic simulation of phase coexistence

We show how an existing concurrent multi-scale method named hybrid particle field-molecular dynamics (hPF-MD) can be adapted to enable the simulation of structure and/or structural dynamics in compressible systems. Implementing such new equations of state (EOS) into hPF-MD, while conserving the efficiency associated with treating intermolecular interactions in a continuum fashion, opens this method up to describe a new class of phenomena in which non-uniform densities play a role, for example, evaporation and crystallization. We carefully consider how compressible hPF-MD compares to its mean-field counterpart for two particular EOS, adopted from the Cell Model for polymers and the Carnahan-Starling expression for hard spheres. Here, we performed a very basic analysis for a single-component system, focusing on the significance of various particle-based parameters and the particle-to-field projection. Our results illustrate the key role of the particle density per field grid cell and show that projection based on a Gaussian kernel is preferred over the standard cloud-in-cell projection. They also suggest that the behavior of hPF-MD close to the critical point is non-classical, i.e., in agreement with a critical exponent for a pure particle description, despite the mean-field origin of the method.