The unit of text written velocity file in vasp.6.5

[jiong_yang1]

Dear all,

It is nice that electrical transport is supported in vasp.6.5, and I have velocity text file printed by changing elphin%write_textvel=.true. in elphon_base.F.

1. What is the unit of the electron group velocity in the "velocity" file?

2. Is there an INCAR tag which control the text print of the velocity file?

Many thanks.

[andreas.singraber]

Hello!

Thank you for your kind words, I will forward them to the developers!

1. The units in the "velocity" file are "eV Ang" (read "eV times angstrom").
2. Yes, there is an INCAR tag

   Code: Select all

   ELPH_WRITE_TEXTVEL = .TRUE./.FALSE.
   

   which switches exactly the boolean variable you set manually in the source code.

I am sorry that this is not yet in our Wiki, we will update this as soon as possible (we cannot do this right now because of technical issues with the Wiki).

All the best,
Andreas Singraber

[jiong_yang1]

Dear Team,

Thank you for your swift response. We have two additional questions regarding VASP 6.5.1:

1. Unit conversion: The group velocity file uses "eV Å" units. Could you confirm the conversion method to SI units (m/s)? Does this require multiplying by the elementary charge and dividing by the reduced Planck constant (i.e., v = [eV Å] × (10-10 m) e / ℏ)?

2. NBANDS discrepancy: During transport calculations, we observe inconsistent band numbers between the OUTCAR (NBANDS=60) and the velocity file (maxband=57). This mismatch persists despite explicit NBANDS=60 setting in INCAR.

Code: Select all

grep NBANDS OUTCAR
   NBANDS = 60
   k-points           NKPTS =    451   k-points in BZ     NKDIM =    451   number of bands    NBANDS=     60
tail velocity 
 48 451  1  3   6.954   0.000
 49 451  1  3   8.383  -0.000
 50 451  1  3   8.388   0.000
 51 451  1  3   8.395  -0.000
 52 451  1  3   8.612   0.000
 53 451  1  3   8.617   0.000
 54 451  1  3   8.622   0.000
 55 451  1  3   9.795  -0.000
 56 451  1  3   9.800  -0.000
 57 451  1  3   9.803  -0.000

Attached are our input/output files. This is an example based on CRTA. We appreciate your guidance on these details.

Many thanks.

[andreas.singraber]

Hello!

1. Yes, I agree that you can use this unit transformation in order to express the given velocities in SI units times ℏ:

   Code: Select all

   1 eV A = 1.602176634 * 10^-19 J  *  10^-10 m
          = 1.602176634 * 10^-29 J m  *  (ℏ / 1.054571817 * 10^-34 J s)
          = 151926.7449 ℏ  m / s
   

2. TL;DR: There is a separate INCAR tag for settings the number of bands for transport calculations: ELPH_NBANDS. If you are interested, now comes the technical background: If NBANDS is not set by the user it is automatically determined by the formula given in the Wiki (max(NELECT/2+NIONS/2,NELECT*0.6)). However, when VASP is executed in parallel the number of bands is increased such that it is an integer-multiple of the number of MPI tasks. This facilitates parallelization over bands. However, for the NSCF calculation before the electron-phonon part there is parallelization over k-points and not over bands. Hence, there is no necessity to increase the value from the default formula. This is why there is a separate tag and the default is not taken from NBANDS.

All the best,
Andreas Singraber