  A series of 20 ps ab initio molecular dynamic simulations of water at ambient
density and temperatures ranging from 300 to 450K are presented. Both
Car-Parrinello (CP) and Born-Oppenheimer (BO) molecular dynamics techniques are
compared for systems containing 54 and 64 water molecules. At 300K, excellent
agreement is found between radial distribution functions (RDFs) obtained with
BO and CP dynamics, provided an appropriately small value of the fictitious
mass parameter is used in the CP simulation. However, we find that the
diffusion coefficients computed from CP dynamics are approximately two times
larger than the corresponding BO simulations for T>400K, where statistically
meaningful comparisons can be made. Overall, both BO and CP dynamics at 300 K
yield overstructured RDFs and slow diffusion as compared to experiment. In
order to understand these discrepancies, the effect of proton quantum motion is
considered with the use of empirical interaction potentials. We find strong
evidence that proton quantum effects may have a larger impact than previously
thought on structure and diffusion of the liquid.
