Dear Pratik, Within the QHA you must do calculations in constant volume. Therefore, it is straightforward to calculate the Helmholtz free energy. To calculate the Gibbs free energy you must know the pressure. To calculate the pressure you must calculate several volumes. You can calculate the Helmholtz free energy at several volumes in some temperature and use the fact that the pressure is minus the partial derivative of the Helmholtz free energy with respect to the volume (to do so, you can fit the Helmholtz free energy to some analytical function, e.g. parabola of Birch-Murnaghan equation of state).
In addition, I want to mention a few things: 1. Recently, a bug in the QHA has been corrected in several structures. It is corrected in the SVN version. 2. In this approach, you must work with hydrostatic pressure. To do so, you need to do relaxation. 3. The pressure you see in the SCF output file is the pressure at zero Kelvin without phonon zero point energy contribution. It is not the pressure at non zero temperatures. 4. There is also a thermal electronic contribution to both Helmholtz free energy and Gibbs free energy. In most cases, it is very small relative to the phonon contribution, but you cannot know for sure until you calculate it. To calculate it, you need to calculate the electron DOS with dos.x. I hope I helped. Uri Argaman Ben-Gurion University Israel -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140405/e52e3958/attachment.html
