Polaron formation energy

[cemeis]

Dear developers,

first of all, I would like to thank you for the ongoing improvement of the code and its features. I would like to report a small bug concerning the computation of the polaron formation energy. I used the flag efermi_read=.true. and set a Fermi energy to a value within the bandgap in the polaron calculation. It led to a change of the electronic contribution to the formation energy by the difference of the Fermi energy to the valence (conduction) band extrema in the hole (electron) polaron calculation, respectively. My suggestion would be to either stop a polaron calculation if efermi-read is set to true or to adjust the equations accordingly to make them independent of the Fermi energy that is set in the input file. If further details or examples are needed, I am happy to provide them.

Best regards,
Christoph Emeis

[jlb]

Dear Christoph,

Thanks for pointing this out. The polaron formation energy is defined as the energy difference with respect to the periodic insulating crystal with one added/removed electron, so efermi_read should be kept to .false. as it is done by default. The code then sets to zero the KS energy of the CBM/VBM for electron/hole polaron calculations. In fact, currently the code prints in the output where the located CBM/VBM is, and it is a good practice to double-check that it is what expected, to make sure that the CBM/VBM k-point is included in the fine grid.

In any case, we will make a fix so that the code overwrites the Fermi energy specified by the user with the CBM/VBM energy if plrn=.true. is activated. We will also add a warning message in the output to let the user know.

Thanks again for spotting this, and let us know if you find any other issues!

Best,
Jon Lafuente-Bartolome