Possibly, here I'm on holiday and I can't really check, bu I think that it is, safer to see this kind of derivative in cartesian coordinates than think what it means to rotate on the basis of the displacements, there could be a d u/d x factor
-- Lorenzo Paulatto Written on a virtual keyboard with real fingers On Sat, 14 Jul 2018, 11:25 JAY Antoine, <antoine....@isae-supaero.fr> wrote: > Dear Lorenzo, > I also forget to precise that as (R0+i*0.01U) I mean the norm of i*0.01*U > which I defined in the 3Nat space as sqrt(sum_n(at_n_X^2+at_n_Y^2+at_nZ^2)) > Do you think this can be another source of error? > > Antoine Jay > > > On Saturday, July 14, 2018 11:09 CEST, "JAY Antoine" < > antoine....@isae-supaero.fr> wrote: > > > > > 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 <paul...@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....@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 >> >> >> >> _______________________________________________ >> users mailing list >> users@lists.quantum-espresso.org >> https://lists.quantum-espresso.org/mailman/listinfo/users > > > > > > > > _______________________________________________ > users mailing list > users@lists.quantum-espresso.org > https://lists.quantum-espresso.org/mailman/listinfo/users
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