  We use first-principles techniques to re-examine the suggestion that
transitions seen in high-P experiments on Mo are solid-solid transitions from
the bcc structure to either the fcc or hcp structures. We confirm that in the
harmonic approximation the free energies of fcc and hcp structures become lower
than that of bcc at P > 325 GPa and T below the melting curve, as reported
recently. However, we show that if anharmonic effects are fully included this
is no longer true. We calculate fully anharmonic free energies of high-T
crystal phases by integration of the thermal average stress with respect to
strain as structures are deformed into each other, and also by thermodynamic
integration from harmonic reference systems to the fully anharmonic system. Our
finding that fcc is thermodynamically less stable than bcc in the relevant
high-P/high-T region is supported by comparing the melting curves of the two
structures calculated using the first-principles reference-coexistence
technique. We present first-principles simulations based on the recently
proposed Z method which also support the stability of bcc over fcc.
