HF Parameters for defects in GaN

[blair_tuttle]

I recently started a project to investigate point defects in GaN. I have been unable to reproduce the hyperfine parameters from previous calculations [PRL 109, 206402 (2012)]. Specifically, my spin density values are much more delocalized especially when taking core polarization effects into account. Attached are the calculations and a summary doc. please advise.

[blair_tuttle]

For more context, the author of the previous HF calculations [PRL 109, 206402 (2012)] mentioned that a relativistic expression for the HF parameters needs to be used for scalar relativistic pseudopotentials. I am not sure how this effects PAW results.

[alexey.tal]

Dear blair_tuttle,

Thank you for your question.

Indeed, I see that the values reported in [PRL 109, 206402 (2012)] differ from the ones you find with VASP. However, I found some traces of HSE calculation in you INCAR, i.e., HFSCREEN = 0.2. But on the other hand you use LHFCALC=.FALSE.. After looking at the paper I can't tell if the results you use as a reference are produced with PBE or HSE. As far as I know, the choice of xc functional can largely affect the hyperfine tensor calculation (see Szász et al., Phys. Rev. B, 075202 (2013)). That would also explain why the spin density is much more delocalized in your case as the PBE tends to delocalized the charge density compared to hybrid functionals. Are you sure that it is the PBE values you are using as a reference and not HSE?

Kind regards,
Alexey

[blair_tuttle]

the tests I presented are for PBE calculations because the 2012 PRL used PBE. However, I have performed both HSE and PBE calculations, which is why the INCAR file has some HSE flags. i assume that using LHFCALC=.FALSE. nullifies the HFSCREEN = 0.2 flag. BTW, the PBE and HSE VASP results are very similar and differ from the 1012 PRL and experiment. The main difference i see between my VASP and the 2012 PRL is that the latter used norm-conserving pseudopotentials and a larger PWQ cutoff. They mentioned the need for a relativistic formula for HF parameters but i did not see any mention of such a formula in the VASP manual. thanks for any help to explain this VASP error.

[alexey.tal]

Could you please share the OUTCARs from the HSE calculations? I don't know yet what causes this discrepancy but I will looking into it.

[blair_tuttle]

Attached is tar file showing PBE and HSE OUTCAR files for the same POSCAR with 299 atoms and with NSW = 0.

[blair_tuttle]

see attached. HSE is more localized than PBE but the 2012 PRL used PBE so why does VASP PBE perform so differently.

[alexey.tal]

Thank you.

PBE and HSE VASP results are very similar and differ from the 1012 PRL and experiment.

I'm a bit confused because I see that the HF parameters from the HSE calculation differ quite substantially from the PBE results.
What results did you find very similar between PBE and HSE?

Here is Total hyperfine coupling parameters from PBE (left) and HSE (right):

Code: Select all

  52     -27.118   -22.872   -27.461      -0.155          |    52     -98.393   -89.829  -101.906      -0.084
  53      -2.641    -2.312    -2.767      -0.119          |    53      -0.646    -0.418    -0.719      -0.317
  54      -1.184    -0.612    -1.251      -0.457          |    54      -2.564    -1.239    -2.659      -0.498
  55     -27.118   -22.872   -27.461      -0.155          |    55     -98.337   -89.730  -101.850      -0.085
  56      -2.641    -2.312    -2.767      -0.119          |    56      -0.648    -0.420    -0.721      -0.316
  57      -0.331    -0.221    -0.337      -0.325          |    57      -0.009     0.002     0.160       0.067
  58      -1.833    -1.646    -1.916      -0.097          |    58      -2.529    -2.270    -2.753      -0.094
  59      -1.077    -1.003    -1.099      -0.067          |    59      -0.234    -0.151    -0.257      -0.321
  60      -0.326    -0.303    -0.344      -0.066          |    60       0.034     0.025     0.075      -0.119
  61      -0.463    -0.324    -0.515      -0.271          |    61      -0.383    -0.259    -0.451      -0.275
  62      -0.443    -0.429    -0.466      -0.028          |    62       0.059     0.043     0.078      -0.201
  63      -0.326    -0.303    -0.344      -0.066          |    63       0.034     0.025     0.075      -0.119

[alexey.tal]

Also, did you compare your band gaps to the ones reported in the paper with PBE and HSE? If there is a discrepancy in the band gaps, it could indicate that something else in these calculations is different, for example, lattice parameter or potentials.