Molecular dynamics simulations and quantum mechanical studies of the hydrogen bond in water cluster systems

Abstract

The formation and persistence of hydrogen bonds are inherently dynamic processes. In this research the technique of molecular dynamics is used to examine the evolution of hydrogen bonds as a function of initial conditions such as temperature, internal energy, and geometry. Clusters of water molecules serve as the prototypical hydrogen-bonded system. Inherent in the molecular dynamics simulations is the choice of the potential energy function. The results of molecular dynamics studies on a semi-empirical quantum mechanical potential surface are compared with simulations using a central force pair potential. In addition to the molecular dynamics calculations, ab initio quantum mechanical calculations of cluster geometry and stability were made for comparison with the predictions of the semi-empirical and empirical potentials. For both potentials used in the molecular dynamics simulations, the hydrogen bond was found to be quite flexible. It is found that the lifetime of a specific hydrogen bond can be substantially less than the lifetime of the cluster. This can be the case even when the average number of hydrogen bonds in the cluster remains constant. The flexibility of the hydrogen bond opens dynamically accessible reaction paths for molecular rearrangement or dissociation. Cooperative motions serve effectively to reduce energy barriers for such reactions. Hence the dynamical simulation can provide more realistic insight into the role of the hydrogen bonds in the process of interest than can examination of the static energy surface. The sensitivity of the results to the details of the potential is discussed. Preparation of the initial state of the molecule or cluster is seen to affect directly the evolution of the system.