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

[Nicola Marzari]

>>Next, if you use the same identical input files, but you put
>>Pb instead of Sr, do you get identical dielectric constants
>>from 1x1x1 and 2x2x1 ?
>>  
>>
> No, for PbTiO3, it's tetragonal structure, not cubic one as SrTiO3. 

I know - I just wanted to make the point that if there were something
odd with the pseudopotential (although I do not know what - a ghost
state ? Even that is unlikey to appear in only one of the two
calculations) you could use the exact SrTiO3 input files, but just put
Pb instead of Sr.

> Note for no reason I used 4x4x4 k-point grid for both unit cell and 
> super cells.
> 
> 1x1x1 unit cell,
>     (       7.981121520       0.000000000       0.000000000 )
>     (       0.000000000       7.981121520       0.000000000 )
>     (       0.000000000       0.000000000       7.542263658 )
> 
> 2x2x1 supercell,
>     (       7.775208874       0.000000000       0.000000000 )
>     (       0.000000000       7.775208874       0.000000000 )
>     (       0.000000000       0.000000000       7.491522972 )
> 
> Maybe I should also try same k-point (4x4x4) sampling for SrTiO3 in two
> different cells, although they are not equivalent, and see what will happen.
> 


OK, this could show you if there were some odd problem with the
calculation with the larger number of kpoints. Also, make sure
oyu have no leftovers from previous calculations, i.e. your directories
are clean.

			nicola

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Prof Nicola Marzari   Department of Materials Science and Engineering
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