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

[Liping YU]

Nicola Marzari wrote:

>
>
>> 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
>> purposes.
>
Acctually, 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 )

(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 ).

>
> Agreed (caveat: if you have a long or large system, e.g.
> a metallic slab or a nanotube, you might need to tighten
> the convergence to -16/-18 - it also depends what do you need
> the phonon for, and how accurately you need them converged.)
>
>>> 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 ).
>>
>
>
> To be on the safe side, use equivalent k-point sampling
> for the two calculations. This means that if in the 1x1x1 unit cell
> (5 atoms) you use, say, a 8x8x8 monkhorst-pack mesh, you should use
> a 4x4x8 mesh in the 2x2x1 supercell. 

That's exactly what I have used, '8 8 8 0 0 0' for 1x1x1 unit cell
and '4 4 8 0 0 0' for 2x2x1 supercell.

I have a feeling (could be wrong) that this might be due to LDA-PP of
Strontium.
Since i did the same calculation for PbTiO3 using the same PPs of Ti and
O as
those in SrTiO3 and it worked well. So I appreciate if someone could
send me
your Sr LDA-PP (if you have one) that i could try.

Thanks again for your suggestion!

Best!
liping