[Pw_forum] Relaxation of low symmetry lattices

[Manoj Srivastava]

Dear Lorenzo and Gabriele, 
 Thank you very much for your suggestions. I will try them out. 

Regards, 
Manoj

On Fri, 9 Oct 2009, Gabriele Sclauzero wrote:

> 
> 
> 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
> > 
> 
> -- 
> 
> 
> o ------------------------------------------------ o
> | Gabriele Sclauzero, PhD Student                  |
> | c/o:   SISSA & CNR-INFM Democritos,              |
> |        via Beirut 2-4, 34014 Trieste (Italy)     |
> | email: sclauzer at sissa.it                         |
> | phone: +39 040 3787 511                          |
> | skype: gurlonotturno                             |
> o ------------------------------------------------ o
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
> http://www.democritos.it/mailman/listinfo/pw_forum
>