[QE-users] ?==?utf-8?q? Recover phonons frequencies from the eigenmodes

[JAY Antoine]

Dear Lorenzo,
As X I convert the atomic displacement to meters:
(R0+i*0.01*U)*alat*au2meters
where alat is the unit cell parameter (in a.u.)
au2meters convert a.u to meters.
R0/i*0.01U is in alat units (cubic cell)

as Y I used the enery obtained in Ry ploted in Joules
so d^2E/dXdX is in kg/s^2.

I think that the difficulty of obtaining someting comparable is in the divison by the masses to obtain a result homogeneous to 1/s^2 (omega^2)

In fact the dynamical matrix is filled by 1/sqrt(M_ati*M_atj) d^2E/dRatidRatj

So the equivalent mass for the eigenmode obtained by diagonalising the matrix must be more complicated than just the reduce mass?

If all my atoms are the same I just have to divide by one mass, but if not....

Antoine





On Saturday, July 14, 2018 10:51 CEST, Lorenzo Paulatto <paulatz at gmail.com> wrote:
 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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