[Pw_forum] error on phonon calculation (root not converged)
giannozz at nest.sns.it
Wed Apr 12 19:33:24 CEST 2006
On Wednesday 12 April 2006 04:39, Liping YU wrote:
> >total energies per cell are not the same. Also eigenvalues are clearly
> >not the same. Your 2x2x1 supercell is not 4 times a 1x1x1 cell.
> Yes. The total energy difference between them is
> -275.21362101 *4 - (-1100.81820918) = 0.03627486 ryd
> What's the reason for this difference?
actually the reason for this difference is more subtle than I
initially though. If you run the 1x1x1 cell with more bands,
or with a small gaussian broadening, you will get exactly
the same results as with the default setting for an insulator
(in fact your system has a sizable gap, ~2 eV). If you do the
same with the 2x2x1 supercell, you find a nonnegligible
difference between the two runs. It looks like the system
with default options for an insulator falls into a metastable
self-consistent solution. Then you will get very strange
phonons and dielectric tensors.
I have never seen anything like this before.
Paolo Giannozzi e-mail: giannozz at nest.sns.it
Scuola Normale Superiore Phone: +39/050-509876, Fax:-563513
Piazza dei Cavalieri 7 I-56126 Pisa, Italy
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