Computer simulations of liquid/vapor interface in Lennard-Jones fluids: Some questions and answers

Abstract

Canonical molecular dynamics ͑MD͒ and Monte Carlo ͑MC͒ simulations for liquid/vapor equilibrium in truncated Lennard-Jones fluid have been carried out. Different results for coexistence properties ͑orthobaric densities, normal and tangential pressure profiles, and surface tension͒ have been reported in each method. These differences are attributed in literature to different set up conditions, e.g., size of simulation cell, number of particles, cutoff radius, time of simulations, etc., applied by different authors. In the present study we show that observed disagreement between simulation results is due to the fact that different authors inadvertently simulated different model fluids. The origin of the problem lies in details of truncation procedure used in simulation studies. Care has to be exercised in doing the comparison between both methods because in MC calculations one deals with the truncated potential, while in MD calculations one uses the truncated forces, i.e., derivative of the potential. The truncated force does not uniquely define the primordial potential. It results in MD and MC simulations being performed for different potential models. No differences in the coexistence properties obtained from MD and MC simulations are found when the same potential model is used. An additional force due to the discontinuity of the truncated potential at cutoff distance becomes crucial for inhomogeneous fluids and has to be included into the virial calculations in MC and MD, and into the computation of trajectories in MD simulations. The normal pressure profile for the truncated potential is constant through the interface and both vapor and liquid regions only when this contribution is taken into account, and ignoring it results in incorrect value of surface tension.

Finite-size scaling study of dynamic critical phenomena in a vapor-liquid transition

“…The difference in the numbers between our study and these previous ones are related to these facts. In fact, the cut-off dependence of the critical temperature is nicely demonstrated by Trokhymchuk and Alejandre [56]. However, we cannot use the information from this work because of the force correction that we use.…”

The nature of the calculation of the pressure in molecular simulations of continuous models from volume perturbations

“…This can have a dramatic effect in the computed values of the pressure or the surface tension in fluid-fluid interfaces as has been pointed out by Trokhymchuk and Alejandre. 32 From our experience, algorithms based on volume perturbations are also found to be slightly less demanding ͑in terms of computing time͒ than typical algorithms used in constantpressure simulations. This not only applies to results obtained with volume-perturbation methods, but also to those obtained via the usual virial route.…”

“…A comparison with the data reported in the literature indicates consistency in the values of the normal and tangential components of the pressure and the corresponding interfacial tension. 32 The dependence of P N , P T , and ␥ on the size and sign of the volume perturbation is shown in Fig. 5 for the ST LJ system at T * = 0.72.…”

Surface tension of the most popular models of water by using the test-area simulation method

“…40 Secondly, the truncation of the potential is known to significantly affect the interfacial properties and different authors typically use different values of the cutoff distance. 41 Also there could be finite-size effects when the area of the interface is too small.…”