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Towards an assessment of the accuracy of density functional theory for first principles simulations of water. II

2004; American Institute of Physics; Volume: 121; Issue: 11 Linguagem: Inglês

10.1063/1.1782074

1520-9032

Eric Schwegler, Jeffrey C. Grossman, François Gygi, Giulia Galli,

Advanced Chemical Physics Studies

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.