Low-pressure phase diagram of crystalline benzene from quantum Monte Carlo

TL;DR

This study uses quantum Monte Carlo and density functional theory to accurately map the low-pressure phase diagram of crystalline benzene, predicting phase transitions and lattice energies consistent with experimental data.

Contribution

It provides benchmark quantum Monte Carlo calculations for benzene's phase diagram, improving accuracy over existing density functional methods.

Findings

Pbca to P21/c phase transition at 2.1 GPa

DMC lattice energy estimate of 50.6 kJ/mol

Phase stability predictions align with experimental results

Abstract

We study the low-pressure (0 to 10 GPa) phase diagram of crystalline benzene using quantum Monte Carlo (QMC) and density functional theory (DFT) methods. We consider the $Pbca$, $P4_32_12$, and $P2_1/c$ structures as the best candidates for phase I and phase II. We perform diffusion quantum Monte Carlo (DMC) calculations to obtain accurate static phase diagrams as benchmarks for modern van der Waals density functionals. We use density functional perturbation theory to compute phonon contribution in the free-energy calculations. Our DFT enthalpy-pressure phase diagram indicates that the $Pbca$ and $P2_1/c$ structures are the most stable phases within the studied pressure range. The DMC Gibbs free-energy calculations predict that the room temperature $Pbca$ to $P2_1/c$ phase transition occurs at 2.1(1) GPa. This prediction is consistent with available experimental results at room temperature. Our DMC calculations show an estimate of 50.6$\pm$0.5 kJ/mol for crystalline benzene lattice energy.