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<div dir="ltr"><font color="#000000" size="3" face="Tahoma">Dear WIEN2k users and authors,</font></div>
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<div dir="ltr"><font face="tahoma">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 </font></div>
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<div dir="ltr"><font face="tahoma"><font face="tahoma">Fx_i</font> = del_E/(2*dx), where del_E = Etot (x_i + dx) - Etot (x_i - dx)</font></div>
<div dir="ltr"><font face="tahoma"></font> </div>
<div dir="ltr"><font face="tahoma">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).
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<div dir="ltr"><font face="tahoma"></font> </div>
<div dir="ltr"><font face="tahoma">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? </font></div>
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<div dir="ltr"><font face="tahoma">Thank you.</font> </div>
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