phonons with hexagonal symmetry and IBRION=6
Posted: Wed Jan 27, 2010 4:35 pm
I am having trouble with the symmetry implementation of finite difference phonon calculations (IBRION=6). The problem is apparent in hexagonal structures (e.g. Mg) but not cubic structures (e.g. Al, Si).
Experimentally, the Gamma-point phonon frequencies for Mg are around 7.3 THz (nondegenerate) and 3.7 THz (2x degenerate). Using IBRION=5 (finite difference without symmetry), 7 (DFPT without symmetry) or 8 (DFPT with symmetry) I consistently find 7.4 THz and 3.8 THz in good agreement with experiment. These values are relatively insensitive to electronic k-point density, PREC settings, and the values of POTIM and NFREE. I’m using PAW_GGA potentials.
When I take IBRION=6 (finite difference with symmetry) I get 7.4 THz for the nondegenerate mode, but the 2x degenerate mode yields frequencies 2.1*i (NFREE=1), 4.8 THz (NFREE=2) and 3.4 THz (NFREE=4).
This may additionally be related to the difficulties being reported by users skaram, forsdan and jsfilhol in recent weeks.
Experimentally, the Gamma-point phonon frequencies for Mg are around 7.3 THz (nondegenerate) and 3.7 THz (2x degenerate). Using IBRION=5 (finite difference without symmetry), 7 (DFPT without symmetry) or 8 (DFPT with symmetry) I consistently find 7.4 THz and 3.8 THz in good agreement with experiment. These values are relatively insensitive to electronic k-point density, PREC settings, and the values of POTIM and NFREE. I’m using PAW_GGA potentials.
When I take IBRION=6 (finite difference with symmetry) I get 7.4 THz for the nondegenerate mode, but the 2x degenerate mode yields frequencies 2.1*i (NFREE=1), 4.8 THz (NFREE=2) and 3.4 THz (NFREE=4).
This may additionally be related to the difficulties being reported by users skaram, forsdan and jsfilhol in recent weeks.