TO The Quantum Espresso Experts,
I was performing a convergence test for the cutoff energy of the SnO2 unit cell
(Rutile) when I stumbled upon something odd during the 'vc-relax; runs. At some
wfc cutoff energies QE does not find the proper symmetries, giving this error:
warning: symmetry operation # N not allowed. fractional translation:
0.5000000 -0.5000000 0.5000000 in crystal coordinates
The program then proceeds to optimize the cell, but does not recognize its
orthogonality and I end up with a parameter matrix that contains very small-
but very present- off-diagonal elements.
Even stranger, the vc-relax calculation recognizes the symmetries and runs
perfectly for a wfc cutoff of 30,35,40, and 55 Ry, but at 25, 45, and 50 Ry the
symmetries aren't found. Also, my rho cutoff is 10*wfc for each of the
calculations. (i.e. 30->300, 35->350, etc.)
I tried simplifying my coordinates, thinking that my precision was overkill,
but that didn't work, and I am thinking it has to do with my pseudopotentials.
I am using the O.pw91-van_ak.UPF included with QE for Oxygen, and
Sn.pw91-n-van.UPF for Tin, which is downloadable from the QE site at:
http://www.quantum-espresso.org/pseudo/1.3/html/Sn.html
Right now I am continuing my studies with a wfc cutoff of 30Ry and a rho cutoff
of 300Ry, since at this energy there was no issue with symmetries or bfgs
convergence and the final parameters weren't that much different from the 55Ry
calculation. However I am curious to know why these energies are causing
problems with the symmetry finder.
Thank you for your time,
Daniel Cellucci
University of Georgia
Physics Department
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