Calculation of excited State frequencies using FERWE and FERDO flags.

[aijitha_john]

Dear all,
We can optimize a specific excited state by using the FERWE and FERDO flags. Has anyone used this approach to calculate frequencies using DFPT? I calculated the excited-state frequencies with the FERDO and FERWE flags but encountered negative frequencies (-11 THz). Is this reasonable?

[fabien_tran1]

Hi,

We would need more detail about your calculation. Could you please provide the input files (but not POTCAR), as well as the OUTCAR file, as explained at forum/viewtopic.php?t=17928?

[aijitha_john]

Dear abien_tran1,
Inupt and outcar files are attached.

[fabien_tran1]

Did you relax the positions of the atoms in the unit cell for this particular excited state? If not, then the forces on the atoms may be non-negligible, leading to negative phonons frequencies.

Besides, note that you are using DFTP, which means that only the phonons at the GAMMA point are calculated.

[aijitha_john]

Dear fabien_tran1,
Yes. I optimized the geometry for the excited state.

[fabien_tran1]

I optimized your structure in the ground state (i.e., FERWE and FERDO are not used), and then applied DFPT. I also obtained negative frequencies and the largest is even 10 times larger in your case. Did you also calculate the phonons frequencies for the ground state and obtained negative frequencies?