[Pw_forum] About q2r.x problem

[Lorenzo Paulatto]

On Sun, 03 Jul 2011 10:03:37 +0200, yhho <yhho at pub.iams.sinica.edu.tw>  
wrote:
> There's a waring message in output file when I was runing the q2r.x.
>
>       fft-check warning: sum of imaginary terms = .1207107E-07

Such a small amount of imaginary components in the dynamical matrices  
should pose no problem.

>      Message from routine matdyn:
>      Z* not found in file AuC444.fc, TO-LO splitting at q=0 will be  
> absent!
>      A direction for q was not specified:TO-LO splitting will be absent
>      Message from routine matdyn:
>      Z* not found in file AuC444.fc, TO-LO splitting at q=0 will be  
> absent!
>      A direction for q was not specified:TO-LO splitting will be absent
>
> Although I still can get the xxx.freq to plot phonon dispersion, but the  
> result was not great.

If your are NOT studying a polar material, there is no need for the  
effective charge (Z*) long range LO-TO splitting: you can safely ignore  
this warning.

> I would like to ask that wether those warning and error are important or  
> not?

I think they are not.

> How could I improve that?

If I understand your input correctly you are studying 2 very distant  
planes of graphene (why 2 of them??); converging graphene's phonons can be  
tricky especially w.r.t. the k-points. I think you may try to reduce the  
smearing and/or use a more efficient smearing scheme, while increasing the  
k-point sampling along x and y. You can also probably reduce the number of  
k-points along the z direction from 4 to 1; I doubt there is any  
significant interaction between planes that are more then 10 bohr apart!


best regards

-- 
Lorenzo Paulatto IdR @ IMPMC/CNRS & Université Paris 6
phone: +33 (0)1 44275 084 / skype: paulatz
www:   http://www-int.impmc.upmc.fr/~paulatto/
mail:  23-24/4é16 Boîte courrier 115, 4 place Jussieu 75252 Paris Cédex 05