[Pw_forum] Relaxation of low symmetry lattices

[Gabriele Sclauzero]

Manoj Srivastava wrote:
> Dear users,
>  Thank you all for quick reply. Through all the replies I am under the
> impression that the forces can never be actually zero, even for perfect
> FCC lattice. I dont know the exact reason yet, whether it is just
> numerical or something bad on my side. So, I would like to give it one
> more shot, and i have some questions on that.
> 
>  Duy Le: I will try using crystal and report to the forum. Although
> relaxation stops after completing one scf cycle, so atoms actually dont
> move, but still nonzero forces.
> 
>  S. Baroni: When you use cubic symmetry, do you get forces on all atoms
> zero. Did you do the calculation or is this your intution? also, can one
> use cubic symmetry for low symmetry unit cell as in my case? 

As I wrote you in my previous mail, it is not that you use cubic symmetry, but you start 
from cubic symmetry operations to find the actual symmetries (by removing those which do 
not hold anymore depending on the lattice and atomic positions).
You could do the exercise to find out by hand what the symmetry should be in your case and 
check that the symmetry operations correspond to those found by pw.x


> 
>  which is also one order of magnitude smaller than before, so i am
> guessing there might me some other parameters that are not set up
> correctly and causing symmetry breaking? Would someone mind to tell me? I
> dont know much about FFT grid, Isn't FFT dimension determined by the KE
> cutoff? Is there anyother way of changing it?

I don't know what the cause could be, anyway, since you seem to be very curious and to 
have some spare time for testing, I can suggest the following: in forces.f90 uncomment the 
section between #if defined (DEBUG) and #endif and look at the various contributions to 
the forces (this is present in the CVS version, I don't know if also in the released one). 
By looking at the single terms you might find why they do not cancel exactly.

Cheers

GS

> 
> Regards, 
> Manoj Srivastava
> University of Florida
> Gainesville, FL
> 

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