[Pw_forum] about phonon and structural optimization

[sun jason]

Hi, eyvaz,

Thank you to answer my questions:)

did you calculate the phonon mode of cubic BN before?

today, I recalculate the phonon using PBE potentials, the result is better 
now, but the LO mode is 1294 cm-1, about 10 cm-1 less than the experimental 
value.

I calculated the LO/TO spliting using q2r.x +  matdyn.x.
                                            ~~~~~~~~~~~~

but the user manual says, the dynmat.x should be use to calculate LO/TO 
spliting and IR.
                            ~~~~~~~~~~~

so which program do you use to get LO/TO spliting?


if you use pwtools dynmat.x, how to use it?

Thank you in advance.



another question is about cell optimization.

2) does pw.x can calculate from the optimized cell automatically? or I 
should read the output file of vc-relax and write the coordinate into the 
input file of pw.x manually? 

3) I perform a vc-relaxtion using PBE USPP, the optimized lattice constants 
is larger than the experimental value. why? it's reasonable?


jason

Message: 6
Date: Wed, 6 Jul 2005 09:51:30 -0700 (PDT)
From: Eyvaz Isaev <eyvaz_isaev at yahoo.com>
Subject: Re: [Pw_forum] about phonon calculation
To: pw_forum at pwscf.org
Reply-To: pw_forum at pwscf.org

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.