MAGMOM
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 , , , 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