  The PHASEGO package extracts the Helmholtz free energy from the phonon
density of states obtained by the first-principles calculations. With the help
of equation of states fitting, it reduces the Gibbs free energy as a function
of pressure/temperature at fixed temperature/pressure. Based on the
quasi-harmonic approximation (QHA), it calculates the possible phase boundaries
among all the structures of interest and finally plots the phase diagram
automatically. For the single phase analysis, PHASEGO can numerically derive
many properties, such as the thermal expansion coefficients, the bulk moduli,
the heat capacities, the thermal pressures, the Hugoniot
pressure-volume-temperature relations, the Gruneisen parameters, and the Debye
temperatures. In order to check its ability of phase transition analysis, I
present here two examples: semiconductor GaN and metallic Fe. In the case of
GaN, PHASEGO automatically determined and plotted the phase boundaries among
the provided zinc blende (ZB), wurtzite (WZ) and rocksalt (RS) structures. In
the case of Fe, the results indicate that at high temperature the electronic
thermal excitation free energy corrections considerably alter the phase
boundaries among the body-centered cubic (bcc), face-centered cubic (fcc) and
hexagonal close-packed (hcp) structures.
