Evidence for stable square ice from quantum Monte Carlo

Recent experiments on ice formed by water under nanoconfinement provide evidence for a two-dimensional (2D) ``square ice'' phase. However, the interpretation of the experiments has been questioned and the stability of square ice has become a matter of debate. Partially this is because the simulation approaches employed so far (force fields and density functional theory) struggle to accurately describe the very small energy differences between the relevant phases. Here we report a study of 2D ice using an accurate wave-function based electronic structure approach, namely diffusion Monte Carlo (DMC). We find that at relatively high pressure, square ice is indeed the lowest enthalpy phase examined, supporting the initial experimental claim. Moreover, at lower pressures, a ``pentagonal ice'' phase (not yet observed experimentally) has the lowest enthalpy, and at ambient pressure, the ``pentagonal ice'' phase is degenerate with a ``hexagonal ice'' phase. Our DMC results also allow us to evaluate the accuracy of various density functional theory exchange-correlation functionals and force field models, and in doing so we extend the understanding of how such methodologies perform to challenging 2D structures presenting dangling hydrogen bonds.