New estimations of vapor density and surface tension of water at low temperatures using scaled model

Abstract

In this work, we were able to estimate the values of critical temperature, surface tension, excess surface entropy/k per particle and vapor density of all of TIP3P, SPCE, TIP4P, and TIP4P 2005 water models, using a scaled model. This model is based on scaling the free energy difference of n-molecules and (n − 1)-molecules plus a probe to the critical temperature. The unitless free energy difference, using the Bennett acceptance ratio versus the number of molecules to the power minus one-third, shows a behavior of straight line for n > 7. The slope of the straight line is proportional to both surface tension and the excess surface entropy/k per particle, while the intercept is proportional to the logarithmic ratio of liquid density to that of vapor density. By calculating the free energy difference for at least three different temperatures and scaling the results to Tc/T − 1, we were able to estimate the critical temperature as well as the surface tension and the vapor density at a temperature range of 200-300 K.