a Fortran code to compute phonon quasiparticle properties and dispersions

Intrinsic thermal shifts of phonon frequencies due to lattice anharmonicity may be significant in solids at high temperatures. As such, the calculation of phonon dispersions incorporating anharmonic effects is critical for predictive studies of vibrational, thermodynamic, and lattice transport properties. Here we introduce the phq code to compute anharmonic phonon dispersions of crystals that combines molecular dynamics (MD) and lattice dynamics calculations. The method invokes the concept of phonon quasiparticles to extract thermal shifts and phonon lifetimes from velocity autocorrelation functions projected into normal modes sampled by MD simulations. With the renormalized frequencies, it is possible to construct an effective harmonic force constant matrix that allows us to calculate the anharmonic phonon dispersion over the whole Brillouin Zone. Due to the nature of phonon quasiparticles, this approach is applicable not only to simply crystals, but also to complex crystal structures with many atoms per primitive cell with extra effort. We demonstrate successful applications of this code to weakly and strongly anharmonic systems. In addition to temperature-dependent anharmonic phonon dispersions, the vibrational entropy and free energy at constant volume can also be obtained.