Pollard Travis, Beck Thomas L
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221, USA.
J Chem Phys. 2014 Jun 14;140(22):224507. doi: 10.1063/1.4881602.
A theoretical analysis of the cluster-pair approximation (CPA) is presented based on the quasichemical theory of solutions. The sought single-ion hydration free energy of the proton includes an interfacial potential contribution by definition. It is shown, however, that the CPA involves an extra-thermodynamic assumption that does not guarantee uniform convergence to a bulk free energy value with increasing cluster size. A numerical test of the CPA is performed using the classical polarizable AMOEBA force field and supporting quantum chemical calculations. The enthalpy and free energy differences are computed for the kosmotropic Na(+)/F(-) ion pair in water clusters of size n = 5, 25, 105. Additional calculations are performed for the chaotropic Rb(+)/I(-) ion pair. A small shift in the proton hydration free energy and a larger shift in the hydration enthalpy, relative to the CPA values, are predicted based on the n = 105 simulations. The shifts arise from a combination of sequential hydration and interfacial potential effects. The AMOEBA and quantum chemical results suggest an electrochemical surface potential of water in the range -0.4 to -0.5 V. The physical content of single-ion free energies and implications for ion-water force field development are also discussed.
基于溶液的准化学理论，对簇对近似（CPA）进行了理论分析。根据定义，质子的单离子水合自由能包括界面电势贡献。然而，结果表明，CPA涉及一个超热力学假设，该假设不能保证随着簇尺寸的增加，能均匀收敛到体相自由能值。使用经典的可极化AMOEBA力场和辅助量子化学计算对CPA进行了数值测试。计算了尺寸为n = 5、25、105的水簇中促晶型Na(+)/F(-)离子对的焓和自由能差。对离液型Rb(+)/I(-)离子对进行了额外计算。基于n = 105的模拟，预测相对于CPA值，质子水合自由能有小的偏移，水合焓有较大偏移。这些偏移是由连续水合和界面电势效应共同作用引起的。AMOEBA和量子化学结果表明水的电化学表面电势在-0.4至-0.5 V范围内。还讨论了单离子自由能的物理内涵及其对离子-水作用力场发展的影响。
