[Pw_forum] RE: Pw_forum digest, Vol 1 #686 - 7 msgs-about phonon calculation

[sun jason]

Much thank to stefano de Gironcoli,



> 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? 

Has the lattice parameter been optimized ? is 50 ryd cutoff sufficient ?
is LDA adequate in this case ?
is 30 cm-1 out of 1300 really bad compared with present
experimental/theoretical accuracy ?
------------------------------
the optimized lattice is not reach the correct phonon modes, yet.
much high cutoff and k-points till to 80 ryd and 16*16*16 are tested. the 
LO mode is about 30 cm-1 smaller than experimental value.
I think 30 cm-1 is a big gap,hehe:) 
the result from direct method provide by professor K.Parlinski is about 
1054 and 1304 cm-1


>
> 2) why the absolute value of effective charge for B atom and N atom is
> not equal?
>
because the ASR is only exactly satisfied in calculations with very
large k-point samplig.
The violation you report is not large and in any case you can impose ASR
in the input of dynmat.x obtaining results equivalent to calculations
with better sampling.

Yes, when I use 16*16*16, they are equal in principle.

stefano de Gironcoli