Applying the Potential Energy Landscape Formalism to a Flexible Water Model

The potential energy landscape (PEL) formalism is a valuable theoretical approach within statistical mechanics to describe supercooled liquids and glasses. Here, we use the PEL formalism and computer simulations to study the statistical properties of the PEL of a flexible water model. We perform extensive molecular dynamics (MD) simulations of H₂O using the flexible q-TIP4P/F water model over a wide range of volumes and temperatures. We find that the PEL of the q-TIP4P/F water model can be accurately described by a Gaussian landscape. Within the Gaussian landscape, we calculate an equation-of-state (EOS) and the find that EOS predicted from the PEL is in excellent agreement with the density (ρ(T)) and pressure (p(V)) obtained from the MD simulations. Using the EOS obtained from the PEL, we estimate the liquid-liquid critical point (LLCP) for q-TIP4P/F H₂O, is located at P_c≈140 MPa, T_c ≈ 195 K, and ρ_c ≈ 1.02 g/cm³, consistent with the data from the MD simulations. Finally, we compare the PEL properties of the flexible q-TIP4P/F water model with those reported previously for rigid water models (SPC/E and TIP4P/2005). Interestingly, we find that the PEL properties for the flexible and rigid water models are very similar.