Dear WIEN2k users and authors, As it stands, the Hellmann-Feynman forces cannot be computed when spin-orbit coupling (SOC) is present. I want to compute the phonon dispersion relations for an fcc cell using PHONON but I want to use SOC forces so I intend to compute the forces on the atoms using the SOC total energies. The force on a single atom along a coordinate direction, say x, will be calculated as
Fx_i = del_E/(2*dx), where del_E = Etot (x_i + dx) - Etot (x_i - dx) Following the above procedure for an fcc cell with 4 inequivalent atoms, 8 total energy calculations will be carried out for each structure (assuming I want forces along the x-direction only). My concern is how to choose a "good value" of dx that will yield forces accurate to within 0.1 mRy/Bohr. My guess is dx = 0.01 Bohr but I would like to seek some input from other users before I proceed with the calculations. Also, should the total energy be computed to a higher degree of accuracy, say 8 or 9 decimal places? Thank you. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20090714/338b7459/attachment.html>
