# fix precession/spin command

## Syntax

fix ID group precession/spin style args

• ID, group are documented in fix command

• precession/spin = style name of this fix command

• style = zeeman or anisotropy

zeeman args = H x y z
H = intensity of the magnetic field (in Tesla)
x y z = vector direction of the field
anisotropy args = K x y z
K = intensity of the magnetic anisotropy (in eV)
x y z = vector direction of the anisotropy


## Examples

fix 1 all precession/spin zeeman 0.1 0.0 0.0 1.0
fix 1 all precession/spin anisotropy 0.001 0.0 0.0 1.0
fix 1 all precession/spin zeeman 0.1 0.0 0.0 1.0 anisotropy 0.001 0.0 0.0 1.0


## Description

Impose a force torque to each magnetic spin in the group.

Style zeeman is used for the simulation of the interaction between the magnetic spins in the defined group and an external magnetic field:

with mu0 the vacuum permeability, muB the Bohr magneton (muB = 5.788 eV/T in metal units).

Style anisotropy is used to simulate an easy axis or an easy plane for the magnetic spins in the defined group:

with n defining the direction of the anisotropy, and K (in eV) its intensity. If K>0, an easy axis is defined, and if K<0, an easy plane is defined.

In both cases, the choice of (x y z) imposes the vector direction for the force. Only the direction of the vector is important; it’s length is ignored.

Both styles can be combined within one single command line.

Restart, fix_modify, output, run start/stop, minimize info:

By default, the energy associated to this fix is not added to the potential energy of the system. The fix_modify energy option is supported by this fix to add this magnetic potential energy to the potential energy of the system,

fix             1 all precession/spin zeeman 1.0 0.0 0.0 1.0
fix_modify      1 energy yes


This fix computes a global scalar which can be accessed by various output commands.

No information about this fix is written to binary restart files.

## Restrictions

The precession/spin style is part of the SPIN package. This style is only enabled if LAMMPS was built with this package, and if the atom_style “spin” was declared. See the Build package doc page for more info.