[Pw_forum] Re: Different Gamma factors for Q-points in the same star

[Paolo Giannozzi]

On Wednesday 31 May 2006 12:23, Andrea Marini wrote:

> So, unless I am doing something wrong in my calculations 
> (perfectly possible) there could be something wrong in the code.

there is. If you perform the calculation of the Delta V and the 
calculation of electron-phonon coefficients in two separate steps,
you get correct results (or, more exactly, results that are not 
obviously wrong). If however you perform the two calculations in 
a single step, you get incorrect results. This is true for all versions
from 2.1 to 3.0 I think: version 2.0 didn't allow the single-step
calculation, version 3.1 is correct in this respect (no warranty that
it is correct in other respects, though).

I think I know why this happen: the small group of q and the crystal
group are stored in the same set of variables, recalculated and 
overwritten 2n+1 times. Since with the old algorithm all serious
electron-phonon calculations needed a two-step calculation 
(phonons and Delta V with a smaller k-point grid, electron-phonon
coefficients with a much larger grid) I hope that nobody published
wrong results obtained with this code.

As a side note: for some reason the Methefessel-Paxton smearing
(ngauss1=1, hardcoded in PH/elphon.f90) was used, but I don't 
see any good reason to prefer it to simple gaussian (ngauss1=0).

Other questions about the symmetrization requires some more
thought. Thank you Andrea for noticing the above problem.

Paolo
-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
Piazza dei Cavalieri 7      I-56126 Pisa, Italy