Efficient computational methods for modeling the effects of water and ions in molecular dynamics simulations are of fundamental importance to gaining a molecular level understanding of biology. However, qualitative differences between kosmotropic and chaotropic properties of ions, which appear to be of considerable importance in biology, are not captured with existing force fields. In particular, none of the existing polarizable and non-polarizable force fields can describe the anomalous behavior of monovalent ions solutions accurately. Specifically, the self-diffusion constant of water decreases with [NaCl] but remain relatively constant with [KCl]. Recent work has attributed this effect to charge transfer between ions and water. We propose a simple charge-transfer method (SCT) for additive force fields in which charge transfer effects are treated effectively at short range (first hydration shell) yet rapidly dying out with distance so that the nominal charges are preserved at long range. The weaker short-range interactions in simulations lead not only to better agreement with experiment of coordination numbers for waters hydrating an ion, but also with the diffusivity. This method is employed with the recently developed single-site multipole (SSMP) non-polarizable model for water, which is based on a multipole expansion of electrostatic potentials and captures properties of water with remarkable accuracy with virtually no additional computational cost.

Physics of the anomalous diffusion coefficients of monovalent ions in water

256th National Meeting (2018)

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