An orientational perturbation theory for pure liquid water

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Abstract
This work develops a statistical mechanical perturbation theory for understanding and quantifying the role of directional hydrogen bonding in pure water at any given temperature or density. A reference fluid has been defined with no orientational preferences, but which reproduces the short-ranged oxygen order as determined by x-ray or neutron diffraction. The orientational anisotropy can be reintroduced by perturbing the reference potential toward a fully coupled water potential; we have developed a new water model, ST4, which provides some noticeable structural improvements over its predecessor, ST2, to provide these anisotropic interactions. Monte Carlo simulations at 25 "C and 1 kg//mass density have been implemented for various values of the coupling parameter to determine the importance of directed hydrogen bonds at various strengths in dictating energetic and structural features of liquid water. We find that virtually full hydrogen bond strength is required to recover the basic structural features of liquid water. We have also evaluated and contrasted the inherent structures (potential energy minima) for the reference fluid and the ST4 model, where we find that hydrogen bonding provides significant structural rigidity to resist vibrational distortion. Furthermore, we show that the ST4 model exhibits bifurcated hydrogen bonds which only occur in local regions of high density, i.e., they are found as tetrahedral network defects. These high density clusters also include tetrahedral oxygen triplets, sometimes linearly hydrogen-bonded, which may well serve as low energy intermediates for flow processes in liquid water.