fix nve/tspin command

Syntax

fix ID group-ID nve/tspin keyword value ...

  • ID, group-ID are documented in fix command

  • nve/tspin = style name of this fix command

  • zero or more keyword/value pairs may be appended

  • keyword = lattice or spin or spinmass

    lattice value = moving or frozen
  moving = integrate the atomic positions and velocities
  frozen = hold the atoms on a fixed lattice
spin value = moving or frozen
  moving = integrate the spins
  frozen = hold the spins fixed
spinmass value = ms
  ms = spin mass of each atom, as a multiple of its atomic mass (adim)

Examples

fix 1 all nve/tspin spinmass 0.0075
fix 1 all nve/tspin lattice frozen spinmass 0.01
fix 1 all nve/tspin lattice moving spin moving

Description

Added in version TBD.

Perform velocity-Verlet integration of auxiliary inertial spin dynamics for equilibrium sampling, as described in (Tspin). Both the direction and magnitude of each spin can evolve according to

\[m_{s,i}\frac{d^2\vec{S}_i}{dt^2} = \vec{F}^m_i = -\frac{\partial U}{\partial\vec{S}_i},\]

where \(\vec{S}_i\) is the full spin vector and \(m_{s,i}\) is the auxiliary spin mass. These dynamics do not describe physical Landau-Lifshitz precession, as integrated by fix nve/spin.

The lattice and spin keywords select whether atomic positions and velocities, or spins, are integrated. Both also accept yes and no. The spinmass keyword sets each atom’s auxiliary spin mass to the specified multiple of its atomic mass. Alternatively, spin masses can be assigned with velocity/tspin or restored from a restart file. If specified here, spinmass overrides previously assigned values at the start of each run.

TSPIN requires the full Cartesian magnetic force, including its longitudinal component. A compatible interaction must encode

\[\vec{fm}_i = \frac{|\vec{S}_i|}{\hbar} \left(-\frac{\partial U}{\partial\vec{S}_i}\right)\]

in the SPIN fm array. Compatible variable-moment potentials such as pair_style deepspin of DeePMD-kit satisfy this requirement. Fixed-modulus SPIN pair styles that do not supply this full gradient are rejected.

Unless the magnetic potential provides a longitudinal restoring force, add a potential such as fix spring/tspin to control the spin magnitude.

The auxiliary spin kinetic energy is not included in the usual thermodynamic quantities ke, etotal, or econserve. See compute temp/tspin for how to monitor the conserved energy in NVE simulations.

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

Spin velocities and masses are saved in binary restart files, but not in data files. Redefine the TSPIN integrator after read_restart to continue from the saved spin state.

No fix_modify options or output quantities are supported. This fix is not invoked during energy minimization.

Restrictions

This fix requires atom_style spin and the SPIN package; see Build package. Dynamic groups and run_style respa are not supported. There is no KOKKOS version, and use inside a KOKKOS build is not supported.

Do not combine this fix with fix nve/spin on the same atoms. With SPIN pair styles, this restriction also applies to disjoint groups.

Magnetic interactions must supply the full Cartesian spin force described above. Compatibility of additional magnetic fixes, such as fix precession/spin, is not checked automatically.

Default

The default keyword values are lattice = moving and spin = moving.

(Tspin) Huang, Bai, Wang, and Xu, Scalable Canonical and Isothermal-Isobaric Sampling of Coupled Spin-Lattice Systems with Machine-Learning Potentials, arXiv:2506.12877 (2025), doi:10.48550/arXiv.2506.12877.