Density fluctuations, solvation thermodynamics, and coexistence curves in grand canonical molecular dynamics simulations

Fluid transport across nanometric channels induced by electric, pressure, and concentration gradients is ubiquitous in biological systems and fosters various applications. In this context, computer simulation setups with well-defined open-boundary equilibrium starting states are essential in understanding and assisting experimental studies. However, open-boundary computational methods are scarce and do not typically satisfy all the equilibrium conditions imposed by reality. Namely, in the absence of external gradients, (1) the system of interest (SoI) must be at thermodynamic and chemical equilibrium with an infinite reservoir of particles; (2) the fluctuations of the SoI in equilibrium should sample the grand canonical ensemble; (3) the local solvation thermodynamics, which is extremely sensitive to finite-size effects due to solvent depletion, should be correctly described. This point is particularly relevant for out-of-equilibrium systems; and (4) finally, the method should be robust enough to deal with phase transitions and coexistence conditions in the SoI. In this study, we demonstrate with prototypical liquid systems embedded into a reservoir of ideal gas particles that the adaptive resolution simulation (AdResS) method, coupled with particle insertion/deletion steps (AdResS+PI), satisfies all these requirements. Therefore, the AdResS+PI setup is suitable for performing grand canonical and stationary non-equilibrium simulations of open systems.