[Pw_forum] Symmetry of IFCs (Harmonic) for Si

[Navaneetha Krishnan]

Hi Stefano,

Thanks for your reply. I am particularly looking to address an anomaly in
my simulation. I've attached my scf input, ph input and q2r input files
along with the generated force constant file.

The force constants are not symmetric (in fact anti-symmetric, for certain
direction pairs) with respect to the displacement directions. For eg., if
you look at line number 280 and line 800, the fcs are supposed to be equal
as they are Phi_{XY} and Phi_{YX} for the same pair of atoms. I have tried
this with several combinations of k-sampling and Q-sampling and I get the
same anti-symetry (except when no. of Q's is very small, like Q=2). I also
used a large ecutwfc so that there is no convergence issue. This problem
was not there with Al lattice. The force constants were symmetric in this
sense. Is there bug in the code or am I making a mistake in one of the
input files? These are not negligible IFCs. Please have a look.

Thanks in advance!

On Mon, Nov 10, 2014 at 2:44 AM, stefano de gironcoli <degironc at sissa.it>
wrote:

>  the IFC should be symmetric w.r.t. exchange of the two atoms and
> displacement directions as a consequence of being second order derivatives.
> other symmetries may apply in specific cases.
> difficult to comment more without more info on what you are looking for.
> stefano
>
>
>
> On 11/10/2014 09:16 AM, Navaneetha Krishnan wrote:
>
>  Hello QE users,
>
>  I was investigating the IFCs generated by q2r.x after scf and ph
> calculations. It looks like the force constant matrix for certain pairs of
> atoms is not symmetric (with respect to the cartesian coordinates). For
> silicon, the IFC matrices have to be symmetric with respect to the
> cartesian coordinates - right? For aluminum, the output is symmetric, as
> expected. Any thoughts?
>
>  Thanks a lot!
>  --Navaneeth
>  Caltech.
>
>
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-- 
Navaneetha Krishnan Ravichandran,
Graduate Student,
Mechanical Engineering,
Caltech.
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