Liquid-vapor equilibrium isotopic fractionation of water: How well can classical water models predict it?

The liquid-vapor equilibrium isotopic fractionation of water is determined by molecular-based simulation, via Gibbs ensemble Monte Carlo and isothermal-isochoric molecular dynamics involving two radically different but realistic models, the extended simple point charge, and the Gaussian charge polarizable models. The predicted temperature dependence of the liquid-vapor equilibrium isotopic fractionation factors for H₂ O¹⁸ H₂ O¹⁶, H₂ O¹⁷ H₂ O ¹⁶, and H² H¹ O^{16 2}H ¹ O¹⁶ are compared against the most accurate experimental datasets to assess the ability of these intermolecular potential models to describe quantum effects according to the Kirkwood-Wigner free energy perturbation h ² -expansion. Predictions of the vapor pressure isotopic effect for the H₂ O¹⁸ H₂ O ¹⁶ and H₂ O¹⁷ H₂ O¹⁶ pairs are also presented in comparison with experimental data and two recently proposed thermodynamic modeling approaches. Finally, the simulation results are used to discuss some approximations behind the microscopic interpretation of isotopic fractionation based on the underlying rototranslational coupling.