| Abstract
| - The ab initio/classical free energy perturbation (ABC−FEP) method proposed previously by Wood et al.[J. Chem. Phys. 1999, 110, 1329] combines the free energy calculated from a classical simulation of anapproximate model with the free energy of perturbing the approximate solute−solvent interactions to abinitio interaction energies. This method was used to calculate the hydration free energies of Na+ and Cl- ata variety of high-temperature state points (973 K with 0.0101, 0.0935, 0.2796 g/cm3 and 723 K with 0.0098,0.0897, 0.5113 g/cm3). The classical simulations were done with an approximate model, previously derivedby fitting ab initio results at 973 K and 0.535 g/cm3. These were followed by perturbation to a QM/MMmodel in which the interactions of Na+ with H2O and Cl- with H2O are calculated by an ab initio method,while the interactions of H2O with H2O are calculated by the fluctuating charge TIP4P−FQ model. Thepairwise ion−water interaction energies are obtained at the MP2/6-311++G(3df,3pd) level and multibodyinteractions at the B3LYP/6-311++G(3df,3pd) level for Na+ and B3LYP/aug-cc-pVDZ level for Cl-. Estimatesof the accuracy of the ab initio methods and of the sampling errors indicate that these results are more reliablethan previous predictions and can be used as benchmarks to assess the accuracy of molecular dynamicssimulations or empirically parametrized equations of state. Extrapolations using various semiempirical modelswere not very accurate at the state points studied. The model of Tanger and Pitzer, which interpolates betweengas-phase mass spectroscopic results and high-density predictions, was very accurate. Born models failed atthe low-density state points, while compressible continuum models were much better. Interpolation orextrapolation of the present results indicates that previous simulations of two different Lennard−Jones pluscharge models have substantial errors at most state points. The approximate models used in the present workperformed reasonably well at all state points, with differences from the ABC−FEP corrected results rangingfrom 0 to 22 kJ/mol.
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