Equilibrium evaporation coefficients quantified as transmission probabilities for monatomic fluids

Equilibrium molecular dynamics (MD) simulations are used to investigate the liquid/vapor interface where particle exchange between the liquid and vapor phase is quantified in terms of the evaporation and condensation coefficient. The coefficients are extracted from MD simulations via a particle counting procedure. This requires defining a vapor boundary position for which we introduce an accurate and robust method and present a comparative study with existing methods from the literature. This novel method relies on the behavior of the flux coefficient within the interphase region by scanning the position of a particle sink boundary from the liquid toward the vapor phase. We find a distinct local maxima is attained on the vapor side of the interphase that is identified as the vapor boundary position based on an interpretation of transmission probability theory and the Kullback-Leibler divergence. The ratio of the evaporation flux to the outgoing flux at this location is defined as the evaporation coefficient. This method retains the simplicity of existing methods but eliminates several disadvantages. We apply this method to MD simulations of monatomic fluids neon, argon, krypton, and xenon. We observe a correlation between the molecular transport parameter appearing in the transmission probability theory and the characteristic interface fluctuation length scale from the capillary wave theory. This gives an expression for the evaporation coefficient that agrees well with values extracted from MD using the particle counting procedure. Compared to existing methods, the evaporation/condensation coefficient is determined more accurately for temperatures between the triple and critical points.