MAGMOM

MAGMOM = [real array]

Default: MAGMOM	= NIONS * 1.0	for ISPIN=2
	= 3 * NIONS * 1.0	for non-collinear magnetic systems (LNONCOLLINEAR=.T.)

Description: MAGMOM Specifies the initial magnetic moment for each atom, if and only if ICHARG=2, or if ICHARG=1 and the CHGCAR file contains no magnetisation density

If one is searching for a spin polarised (ferro- or antiferromagnetic) solution, it is usually safest to start from larger local magnetic moments, because in some cases, the default values might not be sufficiently big. A safe default is usually the experimental magnetic moment multiplied by 1.2 or 1.5. It is important to emphasize that the MAGMOM tag is used only, if the CHGCAR file holds no information on the magnetisation density, and if the initial charge density is not calculated from the wavefunctions supplied in the WAVECAR file. This means that the MAGMOM tag is useful for two kind of calculations

Calculations starting from scratch with no WAVECAR and CHGCAR file.

Calculations starting from a non magnetic WAVECAR and CHGCAR file (ICHARG=1).

Often such calculations converge more reliably to the desired magnetic configuration than calculations of the first kind. Hence, if you have problems to converge to a desired magnetic solution, try to calculate first the non magnetic groundstate, and continue from the generated WAVECAR and CHGCAR file. For the continuation job, you need to set

ISPIN = 2
ICHARG = 1

in the INCAR file.

Starting from VASP.4.4.4, VASP also determines whether the magnetic moments supplied in the MAGMOM line break the symmetry. If they do, the corresponding symmetry operations are removed and not applied during the symmetrization of charges and forces. This means that antiferromagnetic (AFM) calculations can be performed by specifying anti-parallel magnetic moments for the atoms in the cell: The most simple POSCAR-file for a bcc-cell with AFM spin alignment would be

AF
2.8000
 2.80000
 1.00000   .00000   .00000
  .00000  1.00000   .00000
  .00000   .00000  1.00000
 1 1
Carthesian
  .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 as a matter of fact, but it is possible to determine the local magnetic moments by using the LORBIT tag (and if LORBIT<10 , the RWIGS-tag in addition).

In the case of spin-orbit coupling (LSORBIT) the 3D-orientation of the magnetic moments has to be given with respect to the spin-quantisation axis SAXIS.

Related Tags and Sections

ISPIN, ICHARG, LNONCOLLINEAR, LSORBIT, SAXIS, LORBIT, RWIGS

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