[Pw_forum] about phonon calculation

[Eyvaz Isaev]

Hi,

In order to get correct LO mode you should calculate
phonons using dynmat.x from /pwtools directory.

Effective charges for B and N are not equal due to
numerical errors in our computer calculations. 

Bests,
Eyvaz.

--- sun jason <jasonsun98 at hotmail.com> wrote:

> Dear all,
> 
> I executed a test of phonon calculation of cubic BN,
> the input for scf is 
> like this
>
===========================================================================
> &SYSTEM
>                        ibrav = 2,
>                     celldm(1)= 6.792,
>                          nat = 2,
>                         ntyp = 2,
>                      ecutwfc = 50.0 ,
>  /
>  &ELECTRONS
>                     conv_thr = 1.0d-9 ,
>                  mixing_beta = 0.7 ,
>  /
> ATOMIC_SPECIES
>     B   10.81100  B.pz-vbc.UPF 
>     N   14.00674  N.pz-vbc.UPF 
> ATOMIC_POSITIONS 
>     B      0.000000000    0.000000000    0.000000000
>    
>     N      0.250000000    0.250000000    0.250000000
>    
> K_POINTS automatic 
>   6 6 6   1 1 1 
>
===========================================================================
> 
> and the result of dielectric tensor and effective
> charge is like this:
> 
>
===========================================================================
> Dielectric constant in cartesian axis 
> 
>           (       4.637335813       0.000000000     
>  0.000000000 )
>           (       0.000000000       4.637335813     
>  0.000000000 )
>           (       0.000000000       0.000000000     
>  4.637335813 )
> 
>           Effective charges E-U in cartesian axis 
> 
>            atom      1
>           (        1.85986        0.00000       
> 0.00000 )
>           (        0.00000        1.85986       
> 0.00000 )
>           (        0.00000        0.00000       
> 1.85986 )
>            atom      2
>           (       -1.82935        0.00000       
> 0.00000 )
>           (        0.00000       -1.82935       
> 0.00000 )
>           (        0.00000        0.00000      
> -1.82935 )
> 
>
===========================================================================
> and the frequency at gamma point is like this:
>
===========================================================================
> 0.0000    0.0001    0.0001 1055.5528 1055.5528
> 1274.3693
>
===========================================================================
> 
> so my question is:
> 
> 1)why the LO mode is so small(the exact value is
> about 1305 cm-1) when the 
> TO mode is correct? and how to improve it?
> 
> 2) why the absolute value of effective charge for B
> atom and N atom is not 
> equal?
> 
> any help would be appreciate much!
> 
> 
> jason
> 
> 
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