Dielectric and thermodynamic response of a generalized reaction field model for liquid state simulations

Abstract

A generalized reaction field (GRF) model for simulations of liquid water is presented. The method is based on the solution of the Poisson equation for the reaction field acting on a collection of charges in a spherical cavity which is immersed in a continuum dielectric medium. In this model, the reaction field potential arising from the charges in the spherical cavity can be expressed as an infinite series, each term of which is the reaction field arising from a corresponding multipole in the expansion of the charge distribution. The GRF model is used here in simulations of liquid water and of solutions containing one or two ions in water. The dielectric constant computed for liquid water with the generalized reaction field agrees with previous results obtained using the conventional dipolar reaction field. The water orientational correlation functions converge to within 5% of the correct value when the first three terms are included in the multipolar expansion of the reaction field. Thermodynamic results from simulations of a single ion in water in which the GRF was employed demonstrate that the hydration free energy of the ion agrees with the Born hydration energy when the ion is far from the boundary, but when the ion approaches the boundary, there are significant deviations in the free energy. The results of GRF simulations of two ions in water are also reported.