Structure of an accurate ab initio model of the aqueous Na+ ion at high temperatures

The structure of an accurate ab initio model of aqueous sodium ion was calculated at two high temperature state points (573 K, 0.72 g/cm(3) and 723 K, 0.0098 g/cm(3)) by a two-step procedure. First, the structure of an approximate model (the TIP4-FQ model for water and Na-H2O interactions from Liu et al.) was calculated from a molecular dynamics simulation of the model. Then the difference between the structure of the ab initio model and the approximate model was calculated by non-Boltzmann weighting of a sample of 500 configurations taken from the approximate model simulation. Radial distribution functions, average coordination numbers, the distribution of coordination numbers, and an analysis of orientations of water in the first coordination shell are reported for both state points. The average oxygen coordination number (calculated up to the inflection point in the running coordination number) was 4.71 at 573 K and 3.48 at 723 K. Most configurations have four or five coordinated waters at 573 K and three or four at 723 K. At 723 K, waters with their dipole moments pointed at the sodium ion were most common. A wider variety of orientations was found at 573 K and higher density. The difference in structure between the approximate and quantum models was small but significant.