Phonon dispersion in a single-layer material

[teguh2605]

Dear Tadano,

I'm trying to reproduce the phonon dispersion of a single-layer 1T-FeCl2. However, I always obtained negative energy in phonon dispersion, which is incorrect. Before that, I always received a fitting error of force constant above 15%. My question is, does the high error influence the negative energy? if not, could you give a tip on how to remove the negative energy?

Regards,

Teguh

[ttadano]

Hi,

Can you tell us more specifically where in the Brillouin zone the negative mode appears?

[ttadano]

Have you optimized the atomic positions using the supercell? If not, doing so may resolve the issue.

[teguh2605]

Did you mean the original supercell or after displacement?

[ttadano]

The original supercell before displacements.

[teguh2605]

Dear Tadano,

I optimized the original supercell as suggested, but the negative energy still exists (please find the attached file). Is there any suggestion?

Regards,

Teguh

[teguh2605]

Dear Tadano,

I found a confusing case. Previously, the displce.py suggests 6 displacement, as I found in the pattern_HARMONIC file. However, as 6 displacement outputs (disp1.out, ..., disp6.out) were finished, the extract.py always exclude disp1.out as written in the DFSET_harmonic file. even I remove one of the remaining outputs. does the program choose the appropriate ones? because my early calculations include all the outputs.

Regards,

Teguh