  The main idea of the interstitial-ice model is that liquid water consists of
an intact hexagonal lattice with both vacant lattice positions and interstitial
water molecules. Narten, Danford and Levy derived the model parameters from the
X-ray diffraction patterns of liquid water. However, their results were
counter-intuitive: their model had almost no vacancies, the interstitial
concentration was temperature independent and the ice lattice was deformed
resulting in extra anisotropy. To overcome these problems, we refined the model
and derived the model parameters from the proton nuclear magnetic resonance
frequency of pure water. Without any extra anisotropy, a significant
concentration of vacancies was found (3.7% of the lattice positions at
0{\deg}C) and the concentration of both the vacancies and interstitials
increased quasi linear with temperature (0.05 M/{\deg}C). This improved model
successfully explains the thermodynamic data of water and is therefore a
promising candidate for a coherent and analytical water model.
