INSTANTANEOUS NORMAL-MODE ANALYSIS OF LIQUID WATER

We present an instantaneous-normal-mode analysis of liquid water at ro om temperature based on a computer simulated set of liquid configurati ons and we compare the results to analogous inherent-structure calcula tions. The separate translational and rotational contributions to each instantaneous normal mode are first obtained by computing the appropr iate projectors from the eigenvectors. The extent of localization of t he different kinds of modes is then quantified with the aid of the inv erse participation ratio-roughly the reciprocal of the number of degre es of freedom involved in each mode. The instantaneous normal modes al so carry along with them an implicit picture of how the topography of the potential surface changes as one moves from point to point in the very-high dimensional configuration space of a liquid. To help us unde rstand this topography, we use the instantaneous normal modes to compu te the predicted heights and locations of the nearest extrema of the p otential. The net result is that in liquid water, at least, it is the low frequency modes that seem to reflect the largest-scale structural transitions. The detailed dynamics of such transitions are probably ou tside of the instantaneous-normal-mode formalism, but we do find that short-time dynamical quantities, such as the angular velocity autocorr elation functions, are described extraordinarily well by the instantan eous modes.