[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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[Wien] PHONON calculations with spin-orbit coupling

[Seyed Javad Hashemifar]

I think the first criteria is that dx should not be so small that the
resulting dE become comparable or smaller than energy accuracy of your
system. Or other way around, the energy accuracy of your calculation should
be considerably larger than dE, it means that if you reduce dx, then
accuracy (kpoints, rkmax, convergency) should be increased consistently.

Javad Hashemifar

2009/7/14 Atta-fynn, Raymond attafynn at uta.edu

  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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-- 
==
Seyed Javad  Hashemifar
Physics Department, Isfahan University of Technology
84156-83111 Isfahan, Iran
Tel: +98 311 391 2375 Fax:+98 311 3912376
Email:  hashemifar at cc.iut.ac.ir
Homepage:  http://hashemifar.iut.ac.ir
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