[QE-users] Recover phonons frequencies from the eigenmodes

Lorenzo Paulatto paulatz at gmail.com
Sat Jul 14 10:51:32 CEST 2018


Hello Antoine,
Your procedure does not look obviously wrong to me, but you did not say
what X is.

-- 
Lorenzo Paulatto
Written on a virtual keyboard with real fingers

On Sat, 14 Jul 2018, 10:43 JAY Antoine, <Antoine.JAY at isae-supaero.fr> wrote:

> Dear all,
> I would like to (re)obtain the phonons frequencies that I first obtained
> using DFPT but from finite difference.
>
> Lets be R0 the ground state atomic positions and U the normalised atomic
> displacement of a normal mode obtained from DFPT.
> I have calculated the total energy from DFT of 11 structures R0+i*0.01*U
> with i variing from -5 to 5. The so obtained curve is fitted with a second
> order polynom a0+a1*X+a2*X^2, so that I obtain the second order derivative
> of the total energy with respect to the atomic displacements of the studied
> mode: 2*a2. I then divided by the atomic mass (one type of mass) and I
> should obtain the omega^2, but my resulting value is 3 or four times to big.
>
> I use a cubic supercell with one type of atom.
>
> Did someone already performed this kind of work?
> How should I do with differents atomic masses?
>
> Thank you very much for your help,
>
> Antoine Jay
>
>
>
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