A quantum model for water: Equilibrium and dynamical properties

Path integral molecular dynamics and centroid molecular dynamics have been applied to study and modify an empirical flexible model for water, the simple point charge/flexible (SPC/F) model. The quantum structural, thermodynamic and dynamical properties have been calculated and compared to their classical counterparts, as well as to experiment. The path integral molecular dynamics simulations demonstrate that the quantum liquid is less structured and exhibits less hydrogen bonding than its classical analog. Quantization also leads to a lower dielectric constant, relative to the corresponding classical value. Centroid molecular dynamics has been used to calculate single molecule time correlation functions, the Debye dielectric relaxation correlation function, and the power spectrum for the quantum model. These time correlation functions decay more rapidly than the classical ones, indicating that nuclear rotational tunneling occurs in the liquid. The power spectrum of the quantized liquid also exhibits red shifted bend and stretch frequencies relative to the classical model. A modification of the parametrization of the harmonic intramolecular potential for the simple point charge/flexible (SPC/F) model is suggested and tested in order to improve the dielectric properties as well as the values of the vibrational frequencies. The quantum simulation of the modified water model, called SPC/F₂, gives better agreement with the experimental IR spectrum of water and the measured value of the dielectric constant. However, the self-diffusion constant for the modified SPC/F model is still somewhat too large relative to experiment. In addition, the NMR rotational time constant is too small compared to experiment. While these discrepancies leave room for future modifications (e.g., including electronic polarizability), the model represents the first parametrization of an empirical flexible water potential explicitly developed for quantum path integral simulations.