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[Artem Oganov <a.oganov@ucl.ac.uk>]

Dear ABINITioners,

 

Doing convergence tests on phonons at the Gamma-point in stishovite (SiO2) using the GGA (PBE), I found rather large acoustic frequencies of +/-50 cm^-1. Of course, at the Gamma-point they should be zero. These frequencies vary a lot (from real to imaginary), but always remain strongly non-zero when I go to very large plane-wave cutoffs (80 Ha) or very dense k-point meshes. At the same time, frequencies of the optic modes are very well-converged and similar to the experimental values.

Are such things normal? When I do an LDA calculation (with the same GGA-derived pseudopotentials and the same geometry), I find much more tolerable acoustic frequencies.

 

Does anyone have ideas on how serious this error is for generating the IFCs and (if the problem is important) how to cope with it?

 

Thanks a lot,

 

Artem

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Dr. Artem R. Oganov
Research Fellow, Crystallography and Mineral Physics
Dept. of Earth Sciences
University College London
Gower Street
London WC1E 6BT
U.K.
web: http://slamdunk.geol.ucl.ac.uk/~artem
phone: +44 (0)20-7679-3424
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