[Wien] PHONON calculations with spin-orbit coupling

[Atta-fynn, Raymond]

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.
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