  We present the results of Car-Parrinello (CP) simulations of water at ambient
conditions and under pressure, using a rigid molecule approximation. Throughout
our calculations, water molecules were maintained at a fixed intramolecular
geometry corresponding to the average structure obtained in fully unconstrained
simulations. This allows us to use larger time steps than those adopted in
ordinary CP simulations of water, and thus to access longer time scales. In the
absence of chemical reactions or dissociation effects, these calculations open
the way to ab initio simulations of aqueous solutions that require timescales
substantially longer than presently feasible (e.g. simulations of hydrophobic
solvation). Our results show that structural properties and diffusion
coefficients obtained with a rigid model are in better agreement with
experiment than those determined with fully flexible simulations. Possible
reasons responsible for this improved agreement are discussed.
