Understanding the Structure Factor and Isothermal Compressibility of Ambient Water in Terms of Local Structural Environments

We determine contributions to the structure factor of ambient water (modeled using the TIP4P/2005 potential) from molecules with different local structural environments, distinguished based on a tetrahedral order parameter. The small wavenumber behavior of the partial structure factors indicates the presence of effective attractive interactions between molecules with similar local environments and effective repulsive interactions between molecules with different local environments. These effective interactions lead to significant concentration fluctuations under ambient conditions. The concentration fluctuations correlate with density fluctuations, and provide an explanation for the recently reported (Huang et al. Proc. Natl. Acad. Sci. U.S.A. 2009, 36, 15214) increase in the structure factor at small wavenumber, as well as for the well-known anomalous temperature dependence of the isothermal compressibility.