Widom's method for uniform and non-uniform electrolyte solutions

Abstract

We develop and apply test particle methods to calculate chemical potentials in uniform and non-uniform electrolytes using computer simulations. Our techniques are based on the well-known Widom method, but account for non-electroneutral particle fluctuations, implicitly suppressed in a usual Canonical Ensemble Monte Carlo simulation. These fluctuations are shown to contribute greatly to the Widom average for a single ionic species. We compare our results with similar Grand Canonical Monte Carlo simulations for 1:1, 2:1 and 2:2 uniform electrolytes, and 1:1 electrolytes at a charged planar interface. In general, we find good agreement between the methods, all within the statistical fluctuations. The advantages of using the test particle approaches are discussed.