The ABINIT Users Mailing List ( CLOSED )

[Fabien Bruneval <fabien.bruneval@cea.fr>]

Dear Sophie,

This tutorial is rather misleading and should be changed in the future.

The GW calculations needs a k-point grid similar to the one uses in the 
usual DFT calculations.
The only requirement is that the k-point, where you want to know the GW 
band structure, needs to be part of the k-point grid you use.

A shortcoming of the current state of the GW part is that it is not 
really clever in using the symmetries. Therefore, it is generally a good 
policy to
choose a k-point grid that is centered in Gamma or with the usual shift 
0.5 0.5 0.5. Other choices usually perturb the code, which produces tons 
of k-points
in the Brillouin Zone.

If you want to calculate the GW correction in a weird k-point, such as 
[0.3 0.3 0.], I advise to you to calculate the GW corrections in a 
"nice" neighboring point like [0.25 0.25 0], which is part of you 
k-point grid. And then use this GW correction on top of DFT calculation 
made in the precise k-point you wanted.

e_GW(k=0.3) = e_KS(k=0.3) + [ e_GW(k=0.25) - e_KS(k=0.25) ]

This is justified because usually the term within squared brackets is 
rather k-point insensitive.

I'm not sure to be clear enough... If not, just tell me.

Fabien


us_sophie.green@yahoo.com wrote:
> Dear all users,
>
> I looked the tests in abinit about the GW calculations. In the GW corrections
> for the band gap, the convergence tests (including ecutwfn, ecutsigx, nband,
> etc) have done within only one point (the gamma point for Si). I am not sure 
> of
> its accuracy. Besides, the band gap correction for Si is in gamma point 
> (direct
> band gap semiconductor), whether the convergence test on gamma point is
> revelant to the following calculation of the band gap correction in Gamma
> point?
> If I want to calculate the GW correction for the band gap with a nonspecial
> point, for example (0.3 0.3 0), should I test the parameters with the  (0.3 
> 0.3
> 0) point, other than gamma point? The point (0.3 0.3 0) should be included in
> the KSS calculation, then I wrote my input file in the Dataset2 part as
> follows. I give three points including (0.3 0.3 0) ( the other two points are
> interesting points, with which  I will also do GW corrections). I am not sure
> whether it is feasible. 
> Need your help. Thanks in advance.
>
>
> # Dataset2: calculation of kss file
> # Definition of k-points
> kptopt2  0
> nkpt2    3             # A set of 19 k-points containing Gamma
> kpt      0.3  0.3  0
>           0.5  0.5  0
>          -0.5  0.5  0
> #istwfk2  19*1                    # Option needed for Gamma
> istwfk2  3*1
> iscf2    -2             # Non self-consistent calculation
> getden2  -1             # Read previous density file
> nband2   13
> nbandkss2 -1        # Number of bands to store in KSS file
>