A phase transition for solid methane

The phase behaviour for solid methane has been investigated by a lattice sum minimization method. The potential energy between a pair of CH4 molecules as a function of volume is calculated at five different temperatures for both the body centered cubic lattice and the orthorhombic structure using an HFD potential with different parameters for each temperature. The sublimation energy at each temperature, the equation of state (P-V diagram) and the pressure at the phase transition for each temperature are estimated.