[Pw_forum] about phonon calculation

[sun jason]

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