The hydration structure and energetics of biological ions such as Na+, K+, Mg2+, and Zn2+ is still hotly debated, as experiments provide an incomplete picture and computational models have provided inconsistent results. Here, we report extensive QM/MM MD simulations using the most rigorous model that was feasible computationally: an inner sphere containing the ion and 12 molecules using RI-MP2/def2-TZVP while the outer sphere water molecules were represented using the SWM4-NDP Drude polarizable water model. The separation of QM and MM waters was enforced by our Flexible Inner Region Ensemble Separator (FIRES; Rowley, Roux, JCTC, 2012  , 8, 3526). We find coordination numbers of 5.7�5.8 for Na+ and 6.9�7.0 for K+, in good agreement with experimental scattering data and the Drude force field for ions. We recently extended this methodology to divalent ions Mg2+ and Zn2+ to calculate the ion-water RDFs and relative solvation energies using free energy perturbation.

Examining the solvation structure of ions using MP2 QM/polarizable MM molecular dynamics simulations

245th National Meeting (2013)

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