The polarizability of point-polarizable water models: Density functional theory molecular mechanics results

Molecular dynamics (MD) simulations of bulk liquid water at different thermodynamic conditions or of biomolecules in aqueous solution require a molecular mechanics (MM) force field that accounts for the sizable electronic polarizability a of the water molecule. A considerable number of such polarizable water models has been suggested,in the past. Most of them agree that one should employ the experimental value alpha(exp) for the electronic polarizability and compute the induced dipole moment mu(i) through linear response from the electric field E at the position ro of the oxygen atom. However, several more recent models have suggested somewhat smaller values for alpha. Using a hybrid method that combines density functional theory for a selected water molecule with an MM description of its liquid water environment, here we show that the choice of alpha(exp) is solely correct if the induced dipole moment mu(i) is calculated from the average electric field < E > within the volume occupied by the given water molecule. Because of considerable field inhomogencities caused by the structured aqueous environment, the average field < E > is much smaller than the local spot check E(ro). However, as opposed to E(ro), the average field < E > cannot be easily calculated in MM-MD simulations. Therefore, in polarizable MM water models, one should calculate the induced dipole moment mu(i) from E(ro) through the reduced polarizability alpha e(ff) = 0.68 alpha(exp), which then effectively accounts for the inhomogeneities of the electric field within the volume of a water molecule embedded in liquid water.