  We use a hybrid strategy to obtain anharmonic frequency shifts and lifetimes
of phonon quasi-particles from first principles molecular dynamics simulations
in modest size supercells. This approach is effective irrespective of crystal
structure complexity and facilitates calculation of full anharmonic phonon
dispersions, as long as phonon quasi-particles are well defined. We validate
this approach to obtain anharmonic effects with calculations in
MgSiO3-perovskite, the major Earth forming mineral phase. First, we reproduce
irregular temperature induced frequency shifts of well characterized Raman
modes. Second, we combine the phonon gas model (PGM) with quasi-particle
frequencies and reproduce free energies obtained using thermodynamic
integration. Combining thoroughly sampled quasi-particle dispersions with the
PGM we then obtain first-principles anharmonic free energy in the thermodynamic
limit (N right arrow infinity).
