Evidence for Stable Square Ice from Quantum Monte Carlo

TL;DR

This study uses Diffusion Monte Carlo to confirm that square ice is stable at high pressure in 2D, clarifying previous debates and providing insights into phase stability and computational method accuracy.

Contribution

First application of DMC to 2D ice, demonstrating its effectiveness in resolving phase stability debates and evaluating DFT and force field accuracy.

Findings

Square ice is the lowest enthalpy phase at high pressure.

Pentagonal ice is lowest at lower pressures.

Pentagonal and hexagonal ice are degenerate at ambient pressure.

Abstract

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.