[Pw_forum] A question about force constants calculation of Graphene nano ribbon

[Eyvaz Isaev]

Dear Huang,

--- On Fri, 12/18/09, Zhen Huang <huang87 at purdue.edu> wrote:
 
 
> However, I found the software failed to give some of the
> force constant.  

>   1  1
> 252.12788999  0.00000000   -0.00114015  0.00000000 
> -14.77300805  0.00000000
>  -0.00114015  0.00000000  ************  0.00000000  
> -0.06520954  0.00000000
> 

The software, fortunately, did not fail, it just stated that something wrong in your input files which resulted in large IFC components that can not be printed using the f12.8 format. 

Reasons for this:

1. 
>     ecutwfc =16.0,  
>     degauss=0.05
ecutwfc looks very small, I would use 25Ry.
degauss is quite large, usually it is around 0.02
You did not specify ecutrho (8-12 times of ecutwfc) which is important for ultrasoft  PsPs (see your PsP below).

>  C  12.0107  C.pz-rrkjus.UPF

2.
> ATOMIC_POSITIONS
>  C 0.462  0.5  0.000
>  C 0.538  0.5  0.000
>  C 0.462  0.5  0.333
>  C 0.538  0.5  0.333
>  C 0.500  0.5  0.500
>  C 0.577  0.5  0.500
>  C 0.500  0.5  0.833
>  C 0.577  0.5  0.833

You have mistaken here as you left an important parameter. By default atomic positions are in units of the lattice parameter "a". If you visualize your structure you will find very funny picture.

In fact, atomic positions provided are in units of basis vectors, i.e. you should add "crystal" after  "ATOMIC_POSITIONS"

I think this is the main reason.

3.  
>   tr2_ph=1.0d-12,

Try -14.


Bests,
Eyvaz.
 
-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden 
Condensed Matter Theory Group, Uppsala University, Sweden 
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com