  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.
