[Pw_forum] error on phonon calculation (root not converged)

Paolo Giannozzi 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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