Water Phase Diagram from a General-Purpose Atomic Cluster Expansion Potential

TL;DR

This study uses an Atomic Cluster Expansion potential trained on DFT calculations to accurately map the complex phase diagram of water, including various ice polymorphs, across a wide range of temperatures and pressures.

Contribution

The paper introduces a general-purpose ACE potential trained on DFT data that effectively reproduces water's phase diagram and stability regions of ice polymorphs.

Findings

Reproduces main ice polymorphs' stability regions accurately

Maps phase boundaries between 100-500 K and 0-4 GPa

Demonstrates ACE potential's capability for complex phase behavior

Abstract

Water's phase diagram remains one of the most intricate and challenging benchmarks in molecular modeling. In this study, we compute the phase diagram of water using an Atomic Cluster Expansion (ACE) potential trained on density-functional theory (DFT) calculations based on the revPBE-D3 exchange and correlation functional. We compute solid-liquid chemical potential differences and melting points using biased coexistence simulations with the On-the-Fly Probability Enhanced Sampling (OPES) method. Starting from these points, we trace coexistence lines using Gibbs-Duhem integration. This combination of methods allows us to consistently map pressure-temperature phase boundaries and reconstruct the full phase diagram between approximately 100-500 K and 0-4 GPa. The stability regions of the main ice polymorphs (Ih, II, V, VI, and VII) are reproduced in close agreement with experiments. As in earlier studies based on DFT, ice III is metastable and there are systematic shifts of coexistence lines with respect to experimental results. Our results demonstrate the capability of our general-purpose ACE potential to capture the complex phase behavior of water across wide thermodynamic conditions.