Why Is MP2-Water ''Cooler'' and ''Denser'' than DFT-Water?

Density functional theory (DFT) with a dispersionless generalized gradient approximation (GGA) needs much higher temperature and pressure than the ambient conditions to maintain water in the liquid phase at the correct (1 g/cm³) density during first-principles simulations. Conversely, ab initio second-order many-body perturbation (MP2) calculations of liquid water require lower temperature and pressure than DFT/GGA to keep water liquid. Here we present a unifying explanation of these trends derived from classical water simulations using a polarizable force field with different sets of parameters. We show that the different temperatures and pressures between DFT/GGA and MP2 at which the simulated water displays the experimentally observed liquid structure under the ambient conditions can be largely explained by their differences in polarizability and dispersion interaction, respectively. In DFT/GGA, the polarizability and thus the induced dipole moments and the hydrogen-bond strength are all overestimated. This hinders the rotational motion of molecules and requires a higher temperature for DFT-water to be liquid. MP2 gives a stronger dispersion interaction and thus shorter intermolecular distances than dispersionless DFT/GGA, which is why MP2-water is denser than DFT-water under the same external pressure.