Modeling hydration of simple ions on the MP2 potential energy surface with molecular mechanics force fields.

by Wang, Feng; Li, Jicun

With the adaptive force matching (AFM) method, simple mol. mechanics potentials are developed by only fitting to ref. forces calcd. with the MP2 method. Without fitting to any expts., the models developed by AFM gives the hydration free energies of 20 simple salts in excellent agreement with expts. Good agreement is also achieved for ion diffusion consts. and the water diffusion consts. in salt solns. The hydration structure of ions is different for small cations, large cations, or anions. The simple picture of water aligning its dipole to the field of the ion is not always correct. In addn., it is shown that the ion-water cross terms developed by AFM can be used with other water models, not necessarily those developed by AFM. This allows easy incorporations of AFM derived terms to existing mol. mechanics potentials for general simulations of more complex systems.