PNNL

Abstract

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Water molecules in clefts and small clusters are in a significantly different environment than those in bulk water. We have carried out ab initio calculations that demonstrate this in a series of clusters, showing that cooperative effects must be taken into account in the treatment of hydrogen bonds and water clusters in such bounded systems. Hydrogen bonds between water molecules in simulations are treated most frequently by using point-charge water potentials, such as TIP3P or SPC, sometimes with a polarizable extension. These produce excellent results in bulk water, for which they are calibrated. Clefts are different from bulk; it is necessary to look at smaller systems and investigate the effect of limited numbers of neighbors. We start with a study of isolated clusters of water with varying numbers of neighbors of a hydrogen-bonded pair of water molecules. The cluster as a whole is in a vacuum. The clusters are defined so as to provide the possible arrangements of nearest neighbors of a central hydrogen-bonded pair of water molecules. We then scan the length and angles of the central hydrogen bond of the clusters, using density functional theory, for each possible arrangement of donor and acceptor hydrogen bonds on the central hydrogen-bonding pair; the potential of interaction of two water molecules varies with the number of donor and acceptor neighbors. This also involves changes in charge on the water molecules as a function of bond length and changes in energy and length as a function of the number of neighboring donor and acceptor molecules. The energy varies by approximately 6 kBT near room temperature from the highest to the lowest energy when bond length alone is varied, enough to seriously affect simulations.