Quantum, intramolecular flexibility, and polarizability effects on the reproduction of the density anomaly of liquid water by simple potential functions

The ability of simple potential functions to describe the properties of liquid water at a range of thermodynamic state points has been explored. These simple potential function models represent a water molecule by a set of sites, either rigid or flexible relative to each other, that interact with a simple, generally classical, Hamiltonian, which has parameters that are empirically determined. Calculations on several models that include intramolecular flexibility, electronic polarization or quantum mechanical effects have been performed. The consequences of altering these parameters have been systematically examined to determine factors of importance in reproducing properties of pure liquid water. It is found that simple four-site models that incorporate classical intramolecular flexibility or electronic polarization do not improve the description of the density anomaly of liquid water. Quantum statistical mechanical path integral calculations on the classical rigid nonpolarizable TIP5P model [J. Chem. Phys. 112, 8910 (2000)] and the classical flexible nonpolarizable TIP4F model indicate that although quantum mechanical effects destructure the rigid model, they improve the radial distribution and energy distribution properties of the flexible model. In addition, although quantum effects make the density behavior of the rigid model worse, they improve the density behavior of the flexible model. Path integral calculations have also been performed on quantum D2O TIP5P water; this leads to a temperature of maximum density that is higher and to a more structured liquid than results from calculations on quantum H2O TIP5P water. A similar effect is seen with calculations on a five-site rigid model, TIP5P(PIMC), which was parameterized using path integral rather than classical Monte Carlo calculations.