Monte Carlo study of free energy of hydration for Li+, Na+, K+, F−, and Cl− with a b i n i t i o potentials

Abstract

We present extensive calculations of the free energies of hydration for Li⁺, Na⁺, K⁺, F⁻, and Cl⁻ ions evaluated from Monte Carlo simulations of dilute aqueous solutions. Two‐body ab initio potentials, derived from configuration interaction and self‐consistent field calculations, respectively, are used for water–water and ion–water interactions. Free energy differences are calculated with the coupling parameter approach. Different numbers of sampling windows, moves per window, as well as system sizes, are investigated. It is found that: (i) if averages are taken over the forward and the backward directions of perturbation, the errors tend to cancel out; (ii) if the same number of windows and of moves per molecule are used, the spread between the two directions is much smaller for larger systems. Regarding the potentials studied, we find that although the deviation from experimental values goes from about −30% for Cl⁻ to +21% for Na⁺, the overall trend is consistent with experiments and the Born theory.