Applicability of integral equation theory of molecular liquids to the hydration thermodynamic response to charged particles

The thermodynamic response of the solvent water to charged particle insertion evaluated by the integral equation theories (IETs) of molecular liquids was assessed. Molecular Ornstein–Zernike (MOZ) and reference interaction site model (RISM) theories were employed as representative theories of IETs, and the results were compared with molecular dynamics (MD) simulations. Both the MOZ and RISM theories showed comparable agreement with MD simulations in the solute charge range of –1.5 to +1, while a large deviation of the hydration free energy was observed at a high magnitude of solute charge. This discrepancy was identified as resulting from excessive solvent distribution in the first hydration shell, which is attributed to the inadequate description of many-body interactions.