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

Canonical molecular dynamics (MD) and Monte Carlo (MC) simulations for liqu id/vapor equilibrium in truncated Lennard-Jones fluid have been carried out . Different results for coexistence properties (orthobaric densities, norma l and tangential pressure profiles, and surface tension) have been reported in each method. These differences are attributed in literature to differen t set up conditions, e.g., size of simulation cell, number of particles, cu t-off radius, time of simulations, etc., applied by different authors. In t he present study we show that observed disagreement between simulation resu lts is due to the fact that different authors inadvertently simulated diffe rent 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 fo rces, i.e., derivative of the potential. The truncated force does not uniqu ely define the primordial potential. It results in MD and MC simulations be ing performed for different potential models. No differences in the coexist ence properties obtained from MD and MC simulations are found when the same potential model is used. An additional force due to the discontinuity of t he truncated potential at cut-off distance becomes crucial for inhomogeneou s 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 pres sure 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. (C ) 1999 American Institute of Physics. [S0021-9606(99)52441-0].