[Phonopy-users] Phonopy encounters symmetry problem for some supercells

[Tian L. <ti...@ca...>]

Dear all,

I am using phonopy to calculate phonons for a cuprite structure. I got some
negative frequencies in acoustic modes for 2 by 2 by 2 supercell. In order
to check whether it really is an unstable structure, or some numerical
fake, I plan to increase the supercell in one dimension to enhance the
resolution there (I guess this method should work).

Therefore, I generate a 1 by 1 by 10 supercell, phonopy created it well and
printed down the correct symmetry, and VASP did the force calculation well,
and phonopy got the force_sets, so far so good. However, when I tried to
get the dispersion or DOS (I have set DIM= 1 1 10 in the control tab),
bunches of lines saying 'Phonopy encounters symmetry problem', followed by
"Input forces are not enough to calculate force constants, or something
wrong (e.g. crystal structure does not match)."

I am confused about this error because I am using phonopy generated
structure, cuprite is cubic structure, but I don't think supercell has to
be the same in all dimension. I changed to a 2 by 2 by 4 cell, the same
problems still exist.

Could someone help me out, I am using a 1.1 version.

Thanks!

Best,
Tian

-- 
Lan, Tian
Ph.D. Candidate, Department of Applied Physics and Materials Science
California Institute of Technology,
Caltech M/C 138-78, Pasadena, CA, 91125