Generalized free-energy method used to calculate the high-pressure, high-temperature phase transition in solid ${\mathrm{CO}}_2$

Free-energy techniques are generalized to calculate phase transitions in solids at high pressures, using the constant-pressure Monte Carlo method with periodic, deformable boundary conditions. The procedures are then applied to ${\mathrm{CO}}_2$, and a room-temperature phase transition is found at ${\mathit{P}}_{\mathit{t}}$=4.43±0.01 GPa, into an orthorhombic Cmca structure. The volume and entropy change on transition is found to be ΔV=0.35 ${\mathrm{cm}}^3$/mole and ΔS=0.07±0.14 cal/mol K.