Hydrogen bonded clusters in the liquid phase: I. Analysis of the velocity correlation function of water triplets

We present an analysis of hydrogen bonded triplet clusters of water in terms of velocity projections on locally defined molecular coordinates. This makes it possible to perform an approximate normal mode analysis and to establish the relative motions of hydrogen bonded molecules. We find that the interpretation of the velocity autocorrelation function (VACF) of water in terms of stretching and bending motions of hydrogen bonded neighbours is an oversimplification due to cooperative effects in the hydrogen bonded network. Especially, for the peak at we find that it is not only due to O-O-O bending motions in hydrogen bonded groups but also to librational motions of the whole cluster.