Efficient simulation of chemical potentials and phase equilibria in associating fluids: monomer/dimer insertion versus gradual particle insertion in primitive water models

Abstract

Widom's test particle method for simulation of chemical potentials fails for associating (network forming) fluids. Therefore we study more sophisticated insertion methods, such as gradual particle insertion and the recent unbonded particle insertion of Tripathi and Chapman. We model strongly associating fluids by short-ranged primitive models (PMs) due to Nezbeda and co-workers. Recently, we showed that gradual insertion, in principle, is applicable to PM water. Here, we study systematically subcritical chemical potential isotherms, determine vapour–liquid coexistence densities and estimate the critical point density and temperature. For comparison we implement two variants of the Tripathi–Chapman algorithm. In the first case we use as unbonded test particles only particles which remain single molecules (monomers), when inserted and in the second case such test particles which remain as single molecules or form a two-particle cluster with an existing free molecule (monomer/dimer), when inserted. While monomer insertion improves Widom's method slightly, monomer/dimer insertion extends substantially the range of application to the subcritical temperatures studied by gradual insertion. But, for low temperatures monomer/dimer insertion requires extremely long Markov chains and significantly more computer time than gradual insertion. The Tripathi–Chapman algorithm—a direct extension of Widom's method—is much simpler than gradual insertion and therefore should be preferred at supercritical and critical conditions.