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["D. R. Hamann" <drhamann@mat-simresearch.com>]

Dear Ganesh,

The strain perturbation depends on being able to symmetrize by sums over 
subsets of the full BZ k-point mesh in ways that the phonon perturbation 
does not.  The default kshift of 3*0.5 breaks symmetry in most cases.  
If the calculation is organized so that phonon and strain calculations 
are done in different datasets in the same run, this check should be 
performed before you start any serious calculation, if I recall 
correctly.  However, the DDB files just give total energy derivatives 
with respect to perturbations, so  I think you should be able to merge 
calculations done with different sets of k-points.  The elastic 
properties tutorial discusses the k-point symmetry issue for one of the 
cases treated there, but I guess that is easy to overlook.

Good luck,
Don Hamann

PGanesh wrote:
> Hi,
>
> I am computing electric-field and strain perturbations for a 20 atom 
> cell with space group Fm-3m at q=(0,0,0)  ( I am using version 
> 5.4.4).  I have already computed the phonon perturbations 
> successfully.  I get the following error :
>
> symkchk : found identity, with number  1
> symkchk : ERROR -
>   k-point set must have full space-group symmetry
>   there is no match for kpt   1 transformed by symmetry   3
>   Action : change kptopt to 2 or 3 and/or change or use shiftk
>            shiftk = 0 0 0 is always a safe choice.
>
>
> My kptopt is 2.  I changed it to 3, but still get the same error.  I 
> am afraid that if I use the other option of using shiftk=0 0 0, then 
> the I won't be able to merge it with the database of phonon 
> calculations which were done with shift 0.5 0.5 0.5.  I have two 
> questions:
>
> 1> Why am I getting this error for electric-field and strain 
> perturbations, while I didnt get it for phonon pertirbations (i.e. 
> when rfphon =1 )
> 2> Is there a way to get around the problem, so that I don't have to 
> recalculate the phonon perturbations at q=0 0 0 ?
>
> Thanks,
> Ganesh

--
D. R. Hamann
Mat-Sim Research LLC    | Deptartment of Physics
P.O. Box 742            |  and Astronomy
Murray Hill, NJ 07974   | Rutgers University
phone: 908-370-8079     | 732-445-4381

email: drhamann@mat-simresearch.com