I am interesting in the magnetic anisotropy energy (MAE) due to spin-orbit coupling.
According to the force theorem, the MAE can be calculated as the difference in band energies for the two magnetization directions. On the other hand, we can directly compare the total energies corresponding to these directions. Doing VASP calculations I found that the two approaches give different results, even qualitatively. Why?
It could possibly be because the spin-orbit coupling is so strong that it can relax your initial spin setting to a non-collinear configuration, such that the resulting total energies do not correspond anymore to the magnetization directions you're interested in.
I agree with Mohammad Saeed Bahramy and Artur Braun .
For heavy metal, sometimes force theorem fail to describe MAE in heavy metal.
Please be careful about the Fermi level.
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