Thanks for the suggestions and clarifications. I will work on them.
Ganesh
matthieu verstraete wrote:
Good news from Don,
may I add that you should not be using the single 1/2 1/2 1/2 shift in
FCC structures, as the resulting grid is not compatible with the full
set of symmetries. As detailed in one of the tutorials, you should be
using 4 shifts
0.5 0.5 0.5
0.5 0 0
0 0.5 0
0 0 0.5
to get a grid which is shifted but also compatible with all symmetries
(ie applying a symmetry will always move a kpt onto another kpt in the
grid).
The corresponding shifts for hcp bcc etc.. are different, of course. In
most cases it doesn't matter too much if you just integrate over the BZ
(just need to converge the kpt density) but if you start using
symmetries to complete perturbations etc... it does matter, as you have
noticed.
Matthieu
On Thu, Nov 6, 2008 at 11:19 PM, D. R.
Hamann <drhamann@mat-simresearch.com>
wrote:
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
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phone: 908-370-8079 | 732-445-4381
email: drhamann@mat-simresearch.com
--
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Matthieu Verstraete
European Theoretical Spectroscopy Facility (ETSF)
Dpto. Fisica de Materiales,
U. del Pais Vasco,
Centro Joxe Mari Korta, Av. de Tolosa, 72, Phone: +34-943018393
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Mail : matthieu.jean.verstraete@gmail.com
http://www-users.york.ac.uk/~mjv500
--
It is the very mind itself
That leads the mind astray;
Of the mind,
It is essential to lose it,
But, do not be mindless.
(The Unfettered Mind)
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