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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