qha: A Python package for quasi-harmonic free energy calculation for multi-configuration system

The quasi-harmonic approximation (QHA) offers an effective way of calculating the thermodynamic properties of many crystalline solids at high pressures and temperatures. In some cases, e.g., solid solutions or partially disordered systems such as H₂O ice-VII, the system has numerous symmetrically distinct configurations. Here we present a Python package, qha, which can calculate the equation of state and various thermodynamic properties of both single- and multi-configuration crystalline systems in the framework of the QHA. This code has a wide range of applications, including, but not limited to, order-disorder phase transitions ^[1], solid solutions ^[2], complex defect stability ^[3], etc. Apart from its versatility, qha has been tested to be both accurate and computationally efficient. It can also be used as an all-in-one executable or taken apart into stand-alone functions, increasing its usability.

[1] K. Umemoto et al., Chemical Physics Letters. 499, 236–240 (2010).
[2] G. Shukla, R. M. Wentzcovitch, Physics of the Earth and Planetary Interiors. 260, 53–61 (2016).
[3] T. Qin et al., American Mineralogist. 103, 692–699 (2018).