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

[Paolo Giannozzi]

On Sunday 09 April 2006 20:24, Liping YU wrote:

> The same phenomenon still campe up in the self consistent
> calculation of representation # 6 if I only reduce aplpha_mix(1)
> to 0.1 and keep tr2_ph = 1.0e-18. So only using smaller mixing
> parameter doesn't work in this case.
>
> However it works as we expected if I set alpha_mix(1) = 0.1
> and tr2_ph = 1.e-10.

there are 8 orders of magnitude between 10^{-10} (not-so-tight
convergence) and 10^{-18} (execeedingly tight convergence).
Try 10^{-12} and 10^{-14}, it shuod be more than enough for 
all porpouses.

> But the results (here 20-atom 2*2*1 supercell used) don't agree 
> well with those of calculation where 1*1*1 cubic perovskite 
> unit cell (5 atoms inside) are used. For example, the dielectric
> constant of Srtio3, is for 2*2*1 supercell
>
> ( 222.139158023 0.000000000 0.000000000 )
> ( 0.000000000 222.139158023 0.000000000 )
> ( 0.000000000 0.000000000 6.262166130 )
> and for 1*1*1 unit cell
> ( 6.296436141 0.000000000 0.000000000 )
> ( 0.000000000 6.296436141 0.000000000 )
> ( 0.000000000 0.000000000 6.296436141 ).

it seems to me VERY unlikely that such a large difference is due
to insufficient convergence.

P.
--
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
Scuola Normale Superiore    Phone:   +39/050-509876, Fax:-563513 
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