MAGMOM

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MAGMOM = [real array]

Default: MAGMOM	= NIONS * 1.0	for ISPIN=2
	= 3 * NIONS * 1.0	for noncollinear magnetic systems (LNONCOLLINEAR=.TRUE.)

Description: Initial magnetic moment for each atom if no magnetization density is present. Considered when symmetry is determined.

For a magnetic calculation from scratch (ISTART=0), MAGMOM specifies (i) the initial on-site magnetic moment for each atom, and (ii) lowers the symmetry of the system (as of VASP.4.4.4). A magnetic calculation could be either a spin-polarized calculation (ISPIN=2) or noncollinear calculation (LNONCOLLINEAR=T). If the MAGMOM line breaks a symmetry of the crystal, the corresponding symmetry operation is removed and not applied during the symmetrization of, e.g., charges and forces.

When restarting a magnetic calculation, MAGMOM is only used to determine the symmetry of the system and not to set the on-site magnetic moment. Therefore, if you remove the MAGMOM tag before restarting from a converged WAVECAR or CHGCAR, the magnetization is likely to be symmetrized away.

MAGMOM also specifies the initial on-site magnetic moments when a magnetic calculation (ISPIN=2 or LNONCOLLINEAR=T) is started from a non-spin-polarized calculation (ISPIN=1 and LNONCOLLINEAR=F). This implies restarting with ICHARG=1 while the CHGCAR file contains no magnetization density. Starting magnetic calculations from a non-spin-polarized calculation can improve convergence.

The I_CONSTRAINED_M tag can constrain the on-site magnetic moments.

Tip: To converge to the magnetic ground state, we recommend setting the magnetic moments slightly larger than the expected values, e.g., using the experimental magnetic moment multiplied by 1.2 or 1.5.

Important: The final magnetic state strongly depends on the initial values for MAGMOM.^[1] This is true even if no symmetry is used (ISYM=-1), because of the many local minima that most exchange-correlation functionals have within spin-density-functional theory.

Format and basis

For a spin-polarized calculation (ISPIN=2), MAGMOM is a list of NIONS positive or negative values that specify the magnitude and relative orientation of the magnetization on each ion. The on-site magnetic moments have no direction in real space, i.e., no orientation in the lattice.

For noncollinear calculation (LNONCOLLINEAR=T), the on-site magnetic moment is specified by three components for each ion. Without spin-orbit coupling (LSORBIT=False), the total energy depends only on the relative direction of the on-site magnetic moments. Hence, you can give the desired magnetic structure in Cartesian coordinates without considering how the lattice matrix or SAXIS is defined.

With spin-orbit coupling (LSORBIT=True), the three components must be specified in the basis of spinor space that is defined by SAXIS. The default is $\sigma _{1}={\hat {x}}$ , $\sigma _{2}={\hat {y}}$ , $\sigma _{3}={\hat {z}}$ , such that MAGMOM can be given in Cartesian coordinates. The orientation of MAGMOM with respect to the lattice only matters if spin-orbit coupling is included (LSORBIT).

Examples

The most simple input for a bcc cell with AFM spin alignment would be the following.

POSCAR file:

AFM
 2.80000
 1.00000   .00000   .00000
  .00000  1.00000   .00000
  .00000   .00000  1.00000
 1 1
Cartesian
  .00000   .00000   .00000
  .50000   .50000   .50000

with

 ISPIN = 2
 MAGMOM = 1.0 -1.0

specified in INCAR. In a perfectly AFM ordered cell, the total net magnetisation is zero, but the local magnetic moments can be written to the OUTCAR file by setting LORBIT tag (and if LORBIT<10 , the RWIGS tag in addition) in the INCAR file.

If you have problems converging to a desired magnetic solution, try to calculate first the non-magnetic ground state and continue from the generated WAVECAR and CHGCAR. To restart, e.g., a calculation with two atoms that have equally large and antiferromagnetically coupled on-site magnetic moments, you need to set the following in the INCAR file:

ICHARG = 1 
ISPIN = 2 
MAGMOM = m -m

or for a noncollinear

ICHARG = 1
LNONCOLLINEAR = T
MAGMOM = 0 0 m  0 0 -m

Related Tags and Sections

ISPIN, LNONCOLLINEAR, LSORBIT, SAXIS, LORBIT, I_CONSTRAINED_M

Examples that use this tag

↑ Huebsch, M-T and Nomoto, T and Suzuki, M-T and Arita, R,Benchmark for ab initio prediction of magnetic structures based on cluster-multipole theory, Phys. Rev. X 11, 011031 (2021).

[huebsch:prx:11-1] Huebsch, M-T and Nomoto, T and Suzuki, M-T and Arita, R,Benchmark for ab initio prediction of magnetic structures based on cluster-multipole theory, Phys. Rev. X 11, 011031 (2021).

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