PNNL

Abstract

Ab initio molecular dynamics (AIMD) simulations of the hydration shells surrounding the Zn2+ ion are reported for temperatures near 300oC. Simulations using a combined ab initio and classical molecular dynamics (AIMD/MM) approach are also carried out. Both simulations are done with 64 solvating water molecules (~15 ps). The hydration structure predicted from both simulations is found to agree very well with known results from X-ray data. The 1st hydration shell contains six water molecules in an octahedral structure with the hydrogen atoms oriented away from the Zn2+ ion. The six waters in the 1st shell are located at an average distance of 2.44Å. A 2nd hydration shell is observed at 4.59Å. Beyond these shells, the bonding pattern substantially returns to the tetrahedral structure of bulk water. No exchanges are seen between the 1st and 2nd hydrations shells, however many water transfers between the 2nd and outer hydrations shells are observed to occur on a picosecond (ps) time scale via dissociative and associative mechanisms. In general, it is found that the AIMD and AIMD/MM simulations give nearly identical results for structural parameters, EXAFS spectra, and exchange dynamics. These results suggest that AIMD/MM can be used to extend the particle scale and time scale of AIMD simulations of highly charged ions in solution.