The Geometry of Water in Liquid Water from Hybrid Ab Initio-Monte Carlo and Density Functional-Molecular Dynamics Simulations

The geometry of the water molecule in liquid water is computed using a hybrid ab initio molecular orbital-Monte Carlo (MC) simulation method at the HF/3-21G and HF/6-31G* level, and using a hybrid density functional-molecular dynamics (MD) technique with nonlocal exchange and correlation functionals. The OH bond length is calculated to be elongated by 0.005 to 0.008 Å in the liquid compared with the gas phase value, whereas the change in the average HOH angle is minimal with a predicted decrease of 0.2 to 1.3 degrees. The HOH angle exhibits large fluctuations, ranging from ca. 97 to 110 degrees. The condensed phase effect is further characterized by computing the vibrational frequencies through Fourier transform of the normal coordinate autocorrelation functions, and by computing the electric polarization of a QM water in liquid water. The present high-level ab initio MC and MD simulation results should be of use for the development of empirical flexible models for liquid water.