BOND-VALENCE CONTROLS ON LIQUID WATER STRUCTURE: AN AB INITIO MOLECULAR DYNAMICS STUDY

The structure of liquid water is the subject of continuing debate for a number of reasons, including the difficulty in creating and testing suitable force fields for classical molecular dynamics (MD) simulations, and in setting a cutoff length for defining weak O–H bonds. In both cases, classical bond valence theory may provide important constraints. First, the valence sum rule states that the valence of bonds reaching an atom must approximately cancel its atomic valence, which is equal to its formal charge. Bond valences are estimated via exponential expressions fit to thousands of bond lengths from known crystal structures, where the valence sum rule is assumed for the purpose of fitting. Although these expressions are empirically derived, they have been repeatedly shown to be quantitatively accurate for constraining possible crystal structures. If the valence sum rule also applies in liquids, this would impose an important constraint on the numerous proposed models of water structure. Second, bond-valence theorists have proposed an operational definition for bonds that implies that the cutoff length for weak H-bonds should be approximately 289 pm. Applying density functional theory as implemented in the pseudopotential plane-wave module in the NWChem program package, we performed ab initio MD simulations of a system of 32 water molecules. In addition, we performed analogous simulations of carbonic, silicic, and phosphoric acid molecules and ions in baths of 30-31 water molecules. The valence of all bonds reaching the O atoms in these model systems was assessed at each time step of the simulations and averaged. In the water simulation, the valence sum rule was very closely followed, and the average number of weak O–H bonds reaching each O atom was 2.8. In the simulations of solvated oxo-species, the valence sum rule was also closely followed. However, since the total valence of Me–O and strong O–H bonds reaching the O atoms in the oxo-species was quite varied, the average number of weak O–H bonds reaching these O atoms ranged from 1.4 to nearly 6. These results not only confirm that the valence sum rule is likely followed in liquids, but also highlight the possibility of predicting the acid-base properties of, and dielectric response around oxo-species in solution and oxide surfaces.