Phonon Calculation [SOLVED]

[xh59]

Dear all,

I was trying to calculate niobium (bcc structure) phonon dispersion using response function method.
Here I followed tutorial http://www.abinit.org/doc/helpfiles/for-v8.2/tutorial/lesson_rf2.html

So I used trf2_2.in to generate my qpt for trf2_1.in.
When I use ngkpt 4 4 4, everything is right and I can get my phonon dispersion.

But if I used ngkpt 5 5 5, anaddb said body centered lattice must use even ngqpt.

--- !ERROR
message: |
For body-centered lattices, the numbers ngqpt(1:3)
must be even, while they are :
ngqpt(1)*nshiftk = 5
ngqpt(2)*nshiftk = 5
ngqpt(3)*nshiftk = 5
Action : modify ngqpt(1:3) in the input file.
src_file: smpbz.F90
src_line: 330


Could you tell me why it must be even? I thought I can use any grid I want.

Thanks,
Xing

[mverstra]

Hi Xing,

you can use a _centered_ default grid (shiftk 0 0 0 nshiftk 1) of any density, including 5 5 5.

The _optimal_ grid for BCC systems uses 2 shifts and has to be even (see paragraph under shiftk in the input variables web site) - this gives you the best quality integral for a given number of irreducible k-points.

Further, as your q-points have to be differences of k-point grids, using a prime number like 5 or 7 is very constraining. You will have to do ngqpt 5 5 5 as well...

cheers