MAGMOM: Difference between revisions

Latest revision as of 12:27, 26 September 2023

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).

[1]

@@ Line 1: / Line 1: @@
 {{TAGDEF|MAGMOM|[real array]}}
-{{DEF|MAGMOM|NIONS * 1.0|for {{TAG|ISPIN}}{{=}}2|3 * NIONS * 1.0 |for non-collinear magnetic systems ({{TAG|LNONCOLLINEAR}}{{=}}.TRUE.)}}
+{{DEF|MAGMOM|NIONS * 1.0|for {{TAG|ISPIN}}{{=}}2|3 * NIONS * 1.0 |for noncollinear magnetic systems ({{TAG|LNONCOLLINEAR}}{{=}}.TRUE.)}}
-Description: {{TAG|MAGMOM}} Specifies the initial magnetic moment for each atom, if and only if {{TAG|ICHARG}}=2, or if  {{TAG|ICHARG}}=1 and the {{TAG|CHGCAR}} file contains no magnetisation density
+Description: Initial magnetic moment for each atom if no magnetization density is present. Considered when symmetry is determined.
 ----
-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
-.2 or 1.5. It is important to emphasize that the {{TAG|MAGMOM}} tag is
-used ''only'', if
-the {{TAG|CHGCAR}} file holds no information on the magnetisation density, ''and''
-if the initial charge density is not calculated from the wavefunctions supplied in
-the {{TAG|WAVECAR}} file.
-This means that the {{TAG|MAGMOM}} tag is useful for two kind of calculations
-*Calculations starting from scratch with no {{TAG|WAVECAR}} and {{TAG|CHGCAR}} file.
+* For a '''magnetic calculation from scratch''' ({{TAG|ISTART}}=0), {{TAG|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 ({{TAG|ISPIN}}=2) or noncollinear calculation ({{TAG|LNONCOLLINEAR}}=T). If the {{TAG|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.
-*Calculations starting from  a ''non magnetic'' {{TAG|WAVECAR}} and {{TAG|CHGCAR}} file ({{TAG|ICHARG}}=1).
+* When '''restarting a magnetic calculation''', {{TAG|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 {{TAG|MAGMOM}} tag before restarting from a converged {{FILE|WAVECAR}} or {{FILE|CHGCAR}}, the magnetization is likely to be symmetrized away.
-Often such calculations converge more reliably to the desired magnetic
+* {{TAG|MAGMOM}} also specifies the initial on-site magnetic moments when a '''magnetic calculation''' ({{TAG|ISPIN}}=2 or {{TAG|LNONCOLLINEAR}}=T) is '''started from a non-spin-polarized calculation''' ({{TAG|ISPIN}}=1 and {{TAG|LNONCOLLINEAR}}=F). This implies restarting with {{TAG|ICHARG}}=1 while the {{TAG|CHGCAR}} file contains no magnetization density. Starting magnetic calculations from a non-spin-polarized calculation can improve convergence.
-configuration than calculations of the first kind. Hence, if you have
+The {{TAG|I_CONSTRAINED_M}} tag can constrain the on-site magnetic moments.
-problems to converge to a desired magnetic solution, try to calculate
+{{NB|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.|}}
-first the non magnetic groundstate, and continue from the generated
+{{NB|important|The final magnetic state strongly depends on the initial values for {{TAG|MAGMOM}}.{{Cite|huebsch:prx:11}} This is true even if no symmetry is used ({{TAG|ISYM}}{{=}}-1), because of the many local minima that most exchange-correlation functionals have within spin-density-functional theory.|}}
-{{TAG|WAVECAR}} and {{TAG|CHGCAR}} file. For the continuation job, you need to set
+== Format and basis ==
- {{TAG|ISPIN}} {{=}} 2
- {{TAG|ICHARG}} {{=}} 1
-in the {{TAG|INCAR}} file.
-Starting from VASP.4.4.4, VASP also determines whether the magnetic moments supplied in
+* For a spin-polarized calculation ({{TAG|ISPIN}}=2), {{TAG|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.
-the {{TAG|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
+* For noncollinear calculation ({{TAG|LNONCOLLINEAR}}=T), the on-site magnetic moment is specified by three components for each ion. Without spin-orbit coupling ({{TAG|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 {{TAG|SAXIS}} is defined.
-specifying anti-parallel magnetic moments for the atoms in the cell: The most simple  {{TAG|POSCAR}}-file for a bcc-cell with AFM spin alignment
-would be
+* With spin-orbit coupling ({{TAG|LSORBIT}}=True), the three components must be specified in the basis of spinor space that is defined by {{TAG|SAXIS}}. The default is  <math>\sigma_1=\hat x</math>, <math>\sigma_2 =\hat y</math>, <math>\sigma_3 = \hat z</math>, such that {{TAG|MAGMOM}} can be given in Cartesian coordinates. The orientation of {{TAG|MAGMOM}} with respect to the lattice only matters if spin-orbit coupling is included ({{TAG|LSORBIT}}).
- AF
-.8000
+== Examples ==
+* The most simple input for a bcc cell with AFM spin alignment would be the following.
+:{{TAG|POSCAR}} file:
+  AFM
 .80000
 .00000   .00000   .00000
@@ Line 40: / Line 32: @@
     .00000   .00000  1.00000
 1
-  Carthesian
+  Cartesian
     .00000   .00000   .00000
     .50000   .50000   .50000
-with
+:with
    {{TAG|ISPIN}} = 2
    {{TAG|MAGMOM}} = 1.0 -1.0
-specified in {{TAG|INCAR}}.
+:specified in {{FILE|INCAR}}. In a perfectly AFM ordered cell, the total net magnetisation is zero, but the local magnetic moments can be written to the {{FILE|OUTCAR}} file by setting {{TAG|LORBIT}} tag (and if {{TAG|LORBIT}}<10 , the {{TAG|RWIGS}} tag in addition) in the {{FILE|INCAR}} file.
-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 {{TAG|LORBIT}} tag (and if {{TAG|LORBIT}}<10 , the {{TAG|RWIGS}}-tag in addition).
-In the case of spin-orbit coupling ({{TAG|LSORBIT}}) the 3D-orientation of the magnetic moments has to be given with respect to the spin-quantisation axis {{TAG|SAXIS}}.
+* If you have problems converging to a desired magnetic solution, try to calculate first the non-magnetic ground state and continue from the generated {{TAG|WAVECAR}} and {{TAG|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 {{TAG|INCAR}} file:
+ {{TAG|ICHARG}} = 1
+ {{TAG|ISPIN}} = 2
+ {{TAG|MAGMOM}} = m -m
+:or for a noncollinear
+ {{TAG|ICHARG}} = 1
+ {{TAG|LNONCOLLINEAR}} = T
+ {{TAG|MAGMOM}} = 0 0 m  0 0 -m
 == Related Tags and Sections ==
 {{TAG|ISPIN}},
-{{TAG|ICHARG}},
 {{TAG|LNONCOLLINEAR}}, {{TAG|LSORBIT}}, {{TAG|SAXIS}},
-{{TAG|LORBIT}}, {{TAG|RWIGS}}
+{{TAG|LORBIT}},
+{{TAG|I_CONSTRAINED_M}}
 {{sc|MAGMOM|Examples|Examples that use this tag}}
 ----
-[[The_VASP_Manual|Contents]]
-[[Category:INCAR]][[Category:Magnetism]]
+[[Category:INCAR tag]][[Category:Magnetism]][[Category:Symmetry]]