Computation of the pK of liquid water using coordination constraints

The equilibrium constant for autodissociation of water is computed by imposing constraints on proton coordination number in an ab initio molecular dynamics (MD) simulation of the liquid. Dissociation is enforced by gradually decreasing the number of hydrogen atoms coordinated with a selected oxygen to one, transforming a water molecule into a hydroxyl ion. Alternatively, a hydronium ion is created by increasing the proton coordination to three. These two pathways are compared to a previous ab initio MD study by Trout and Parrinello [B.L. Trout, M. Parrinello, Chem. Phys. Lett. 288 (1998) 343] who used a constraint on OH bond length. The resulting numerical estimates of pK(w) = 13 +/- 1 are in good agreement. Tt is shown that coordination constraints are more effective for stabilization of the highly activated and short lived solvent separated ion pairs that form in the advanced phase of the reaction. A predominant solvent separated ion pair structure is found to have a double pentameric structure. (C) 2000 Elsevier Science B.V. All rights reserved.