Ab Initio Molecular Dynamics Study of Formate Ion Hydration

We perform ab initio molecular dynamics simulations of the aqueous formate ion. The mean number of water molecules in the first solvation shell, or the hydration number, of each formate oxygen is found to be consistent with recent experiments. Our ab initio pair correlation functions, however, differ significantly from many classical force field results and hybrid quantum mechanics/molecular mechanics predictions. They yield roughly one less hydrogen bond between each formate oxygen and water than force field or hybrid methods predict. Both the BLYP and PW91 exchange correlation functionals give qualitatively similar results. The time dependence of the hydration numbers are examined, and Wannier function techniques are used to analyze electronic configurations along the molecular dynamics trajectory.