<i>pgm</i>: A Python package for free energy calculation

The quasi-harmonic approximation (QHA) is a powerful method for computing free energy and thermodynamic properties of materials at high pressures (P) and temperatures (T). However, anharmonicity, electronic excitations in metals, or both, introduce an intrinsic T-dependence on the phonon frequencies, which makes the QHA inadequate. Here we present a Python package, pgm, for free energy and thermodynamic property calculations. It is based on the concept of phonon quasiparticles and the phonon gas model (PGM). The free energy is obtained by integrating the entropy, which can be readily calculated for a system of phonon quasiparticles. This method is useful for obtaining free energy in anharmonic insulators and harmonic or anharmonic metals. Calculations of thermodynamic properties and equations of state are also implemented in the code. We demonstrate successful applications of pgm to hcp-iron (ε-Fe) at extreme conditions and cubic CaSiO₃-perovskite, a strongly anharmonic system.