[Pw_forum] help entreaty
Paolo Giannozzi
giannozz at nest.sns.it
Tue Dec 9 14:56:07 CET 2003
On Tuesday 09 December 2003 11:43, sprokuda wrote:
> [...] I get from time to time negative frequencies from PWscf.
> I have no idea what to do futher in this concern.
from the last version of the manual:
* ph.x does not yield acoustic modes with omega(q=0) = 0
This may not be an error: the Acoustic Sum Rule (ASR) is never
exactly verified, because the system is never exactly translationally
invariant as it should be (see the discussion above).
The calculated frequency of the acoustic mode is typically less
than 10 cm^{-1}, but in some cases it may be much higher,
up to 100 cm^{-1}. The ultimate test is to diagonalize the
dynamical matrix with program "dynmat.x", imposing the ASR.
If you obtain an acoustic mode with a much smaller frequency
(let us say < 1 cm^{-1}) with all other modes virtually unchanged,
you can trust your results.
* ph.x yields really lousy phonons, with bad or negative frequencies or
wrong symmetries or gross ASR violations.
Possible reasons:
- Wrong data file file read.
- Wrong atomic masses given in input will yield wrong frequencies
(but the content of file "fildyn" should be valid, since the force
constants, not the dynamical matrix, are written to file).
- Convergence threshold for either SCF ("conv_thr") or phonon
calculation "tr2_ph") too large (try to reduce them),
- Maybe your system does have negative or strange phonon
frequencies, with the approximations you used. A negative frequency
signals a mechanical instability of the chosen structure. Check that
the structure is reasonable, and check the following parameters:
- The cutoff for wavefunctions, "ecutwfc"
- For US PP: the cutoff for the charge density, "ecutrho"
- The k-point grid, especially for metallic systems!
Paolo
--
Paolo Giannozzi e-mail: giannozz at nest.sns.it
Scuola Normale Superiore Phone: +39/050509412
Piazza dei Cavalieri 7 Fax: +39/050509417, 050563513
I-56126 Pisa, Italy Office: Lab. NEST, Via della Faggiola 19
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