<html>Dear Lorenzo,<br />As X I convert the atomic displacement to meters:<br />(R0+i*0.01*U)*alat*au2meters<br />where alat is the unit cell parameter (in a.u.)<br />au2meters convert a.u to meters.<br />R0/i*0.01U is in alat units (cubic cell)<br /><br />as Y I used the enery obtained in Ry ploted in Joules<br />so d^2E/dXdX is in kg/s^2.<br /><br />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)<br /><br />In fact the dynamical matrix is filled by 1/sqrt(M_ati*M_atj) d^2E/dRatidRatj<br /><br />So the equivalent mass for the eigenmode obtained by diagonalising the matrix must be more complicated than just the reduce mass?<br /><br />If all my atoms are the same I just have to divide by one mass, but if not....<br /><br />Antoine<br /><br /><br /><br /><br /><br />On Saturday, July 14, 2018 10:51 CEST, Lorenzo Paulatto <paulatz@gmail.com> wrote:<br /> <blockquote type="cite" cite="CAG+GtJdBb+ST7SLnCmicOhbc-fB6UJuFyd0ad0XM929O-XvwYg@mail.gmail.com"><div dir="auto">Hello Antoine, <div dir="auto">Your procedure does not look obviously wrong to me, but you did not say what X is. <br /> <div data-smartmail="gmail_signature" dir="auto">--<br />Lorenzo Paulatto<br />Written on a virtual keyboard with real fingers</div></div></div> <div class="gmail_quote"><div dir="ltr">On Sat, 14 Jul 2018, 10:43 JAY Antoine, <<a href="mailto:Antoine.JAY@isae-supaero.fr">Antoine.JAY@isae-supaero.fr</a>> wrote:</div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Dear all,<br />I would like to (re)obtain the phonons frequencies that I first obtained using DFPT but from finite difference.<br /><br />Lets be R0 the ground state atomic positions and U the normalised atomic displacement of a normal mode obtained from DFPT.<br />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.<br /><br />I use a cubic supercell with one type of atom.<br /><br />Did someone already performed this kind of work?<br />How should I do with differents atomic masses?<br /><br />Thank you very much for your help,<br /><br />Antoine Jay<br /><br /><br /><br /> _______________________________________________<br />users mailing list<br /><a target="_blank" rel="noreferrer" href="mailto:users@lists.quantum-espresso.org">users@lists.quantum-espresso.org</a><br /><a rel="noreferrer noreferrer" target="_blank" href="https://lists.quantum-espresso.org/mailman/listinfo/users">https://lists.quantum-espresso.org/mailman/listinfo/users</a></blockquote></div></blockquote><br /><br /><br /> </html>