[Pw_forum] Symmetry of IFCs (Harmonic) for Si

[stefano de gironcoli]

dear Navaneeth,
     as noted previously IFC should be symmetric w.r.t exchange of the 
two atoms. this include the vector R that joins them.
     in your case  IFC    Phi(na=1,nb=1)_yx(R=a1)  == 
Phi(nb=1,na=1)_xy(R=-a1) because when you exchange the two atoms you 
need to invert the connecting vector as well.
     indeed  line 280 is equal to line 802 and line 282 is equal to 800 
as they should.

stefano


On 11/11/2014 08:23 AM, Navaneetha Krishnan wrote:
> 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 <mailto: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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>
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>
>
> -- 
> Navaneetha Krishnan Ravichandran,
> Graduate Student,
> Mechanical Engineering,
> Caltech.
>
>
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