BSE (or TDHF LMODELHF) calculation questions

[hong_tang1]

Dear VASP team,

I am calculating a 2D monolayer material with TDHF LMODELHF. I calculated for q=0 (1st k point ) and q= non zero (the 4th k point). The ground state is calculated with SCAN+SOC+U and the ground state is a small gap (about 60 meV ) indirect semiconductor.

The question is:
1. I used vasp.5.4.4, I saw in this link : https://www.vasp.at/wiki/index.php/Beth ... lculations, It says " This feature is only supported as of VASP.6 (in VASP.5 the feature can be enabled, but the results are erroneous). ". Could you please explain a little more about what results are erroneous?

2. Please see the attached results from my calculations.
For 1st k point, which is q=0. the calculated first exciton energy is "1BSE eigenvalue 0.00003109 IP-eigenvalue: 0.00002779". These results look not correct. I checked that the direct gap at the Gamma point from the ground state calculation is about 700 meV. However, the 1st exciton energy is only 0.00003109 eV, which is incredibly small. I also checked the vasprun.xml file for the calculated dielectric functions (real and imaginary parts). epsilon_zz is different from epsilon_xx and epsilon_yy, which is kind of making sense, since my monolayer is placed in the xy plane, z is the out-of-plane direction. However, since z direction is mostly vacuum, so, do we expect epsilon_zz is very close to 1?

3. For the 4th k point result, the 1st exciton energy " 1BSE eigenvalue 0.40759190 IP-eigenvalue: -0.00001326 ". 0.40759190 eV makes sense. However, the "IP-eigenvalue: -0.00001326 " is negative, which is weird. However, if you check the vasprun.xml for dielectric function data, this time, epsilon_xx, epsilon_yy, epsilon_zz are the same, which does not look right.


Please help me for those questions.
Appreciate your help very much!

Best,
Hong

[michael_wolloch]

Dear Hong,

due to time constraints, we are not able to advise on how to set parameters to get correct results for specific calculations. Especially for outdated versions of VASP. Thus I have moved your post to the "users for users" section and invite the community to participate in the discussion and provide ideas and recommendations.

Indeed dielectric-dependent hybrid functionals have been available since version 5.2, but until VASP 6 there was an error in the gradient implementation. You can get a bit of information and the links to the relevant papers on our wiki on the LMODELHF page.

I hope that information can help you get closer to a solution for your issue,
Michael