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

Paolo Giannozzi giannozz at nest.sns.it
Wed Apr 12 10:26:56 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? Is it due to PP of Sr?

no, it is due to either a structural difference, or to a different level 
of approximation (k-points or something else).

> If this is true, then as paolo said, eigenvalues are not the same and
> I guess band gaps of them might be either not same (need to check).
> So can this possible bandgap difference cause to the big difference for
> dielectric constant in xx and yy in two cells?

if you are close to a metallic state, small bandgap differences
can do a large difference in the dielectric constant

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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