Supported only by VASP.4.5 and on. THIS FEATURE IS IN LATE BETA STAGE (BUGS ARE POSSIBLE).
LSORBIT = .TRUE. switches on spin-orbit coupling and automatically sets LNONCOLLINEAR= .TRUE.. This option works only for PAW potentials and is not supported by ultrasoft pseudopotentials. If spin-orbit coupling is not included, the energy does not depend on the direction of the magnetic moment, i.e. rotating all magnetic moments by the same angle results in principle exactly in the same energy. Hence there is no need to define the spin quantization axis, as long as spin-orbit coupling is not included. Spin-orbit coupling however couples the spin to the crystal structure. Spin orbit coupling is switched on by selecting
LSORBIT = .TRUE. SAXIS = s_x s_y s_z (quantisation axis for spin)where the default for SAXIS=
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To initialise calculations with the magnetic moment
parallel to a chosen vector , it is therefore possible to either specify
(assuming a single atom in the cell)
MAGMOM = x y z ! local magnetic moment in x,y,z SAXIS = 0 0 1 ! quantisation axis parallel to zor
MAGMOM = 0 0 total_magnetic_moment ! local magnetic moment parallel to SAXIS SAXIS = x y z ! quantisation axis parallel to vector (x,y,z)Both setups should in principle yield exactly the same energy, but for implementation reasons the second method is usually more precise. The second method also allows to read a preexisting WAVECAR file (from a collinear or non collinear run), and to continue the calculation with a different spin orientation. When a non collinear WAVECAR file is read, the spin is assumed to be parallel to SAXIS (hence VASP will initially report a magnetic moment in the z-direction only).
The recommended procedure for the calculation of magnetic anisotropies is therefore:
LSORBIT = .TRUE. ICHARG = 11 ! non selfconsistent run, read CHGCAR SAXIS = x y z ! direction of the magnetic field NBANDS = 2 * number of bands of collinear runVASP reads in the WAVECAR and CHGCAR files, aligns the spin quantization axis parallel to SAXIS, which implies that the magnetic field is now parallel to SAXIS, and performs a non selfconsistent calculation. By comparing the energies for different orientations the magnetic anisotropy can be determined. Please mind, that a completely selfconsistent calculation (ICHARG= 1) is in principle also possible with VASP, but this would allow the the spinor wavefunctions to rotate from their initial orientation parallel to SAXIS until the correct groundstate is obtained, i.e. until the magnetic moment is parallel to the easy axis. In practice this rotation will be slow, however, since reorientation of the spin gains little energy. Therefore if the convergence criterion is not too tight, sensible results might be obtained even for fully selfconsistent calculations (in the few cases we have tried this worked beautifully).