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

[Paolo Giannozzi]

On Monday 10 April 2006 17:35, Liping YU wrote:

> Actually, for 1e-18, though unconvergence message came up
> in phonon calculation, the dielectric constant I got (see below) 
> was pretty close to 1.0e-10  in the same 2*2*1 supercell.
>
> (2x2x1, tr2_ph=1.0e-18, alpha_mix=0.1)
> ( 222.132924025 0.000000000 0.000000000 )
> ( 0.000000000 222.132924025 0.000000000 )
> ( 0.000000000 0.000000000 6.253206744 )
>
> (2x2x1, tr2_ph=1.0e-10, apha_mix=0.1)
> ( 222.139158023 0.000000000 0.000000000 )
> ( 0.000000000 222.139158023 0.000000000 )
> ( 0.000000000 0.000000000 6.262166130 )

so it is not a problem of convergence. Such large values of the
dielectric constants mean that the gap is very small.

> (1x1x1, tr_ph=1.0e-18, alpha_mix=0.7)
> ( 6.296436141 0.000000000 0.000000000 )
> ( 0.000000000 6.296436141 0.000000000 )
> ( 0.000000000 0.000000000 6.296436141 ).

If you do a supercell calculation with same atomic positions and 
equivalent k-point grid as in the original cell, you should get the 
same Kohn-Sham eigenvalues (refolded into the appropriate 
k-points) and four time the energy of the original cell. If you don't, 
please check very carefully your data.

Paolo
-- 
Paolo Giannozzi             e-mail:  giannozz at nest.sns.it
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