Theoretical investigation of methane under pressure

TL;DR

This study uses molecular dynamics and metadynamics simulations to investigate the phase behavior of methane under pressures up to 25 GPa, revealing phase transitions and validating empirical potentials with ab initio methods.

Contribution

It provides a detailed computational analysis of methane's phase transitions under pressure, including phase boundary estimations and validation of simulation methods.

Findings

Fcc to hcp transition at ~10 GPa and 150 K

B phase transforms to monoclinic phase above 20 GPa

Phase boundaries are higher than spectroscopic estimates

Abstract

We present computer simulations of liquid and solid phases of condensed methane at pressures below 25 GPa, between 150 and 300 K, where no appreciable molecular dissociation occurs. We used molecular dynamics (MD) and metadynamics techniques, and empirical potentials in the rigid molecule approximation, whose validity was confirmed a posteriori by carrying out it ab initio MD simulations for selected pressure and temperature conditions. Our results for the melting line are in satisfactory agreement with existing measurements. We find that the fcc crystal transforms into a hcp structure with 4 molecules per unit cell (B phase) at about 10 GPa and 150 K, and that the B phase transforms into a monoclinic high pressure phase above 20 GPa. Our results for solid/solid phase transitions are consistent with those of Raman studies but the phase boundaries estimated in our calculations are at higher pressure than those inferred from spectroscopic data.