[lammps-users] what is the unit of magnetic spin in LAMMPS ?

Dear LAMMPS users,

I want to perform simulations for magnetic spin dynamics.

As it is said in the manual that the exchange interaction between two magnetic spins is given by

H = -JS1S2

S1 and S2 are two unit vectors representing the magnetic spins.

As I understand, S1 (or S2) only represents the vector direction of spin, whose length is one. It does not have unit.

Am I right?

Best wishes,


Dear Wei,

You might find detailed information in the publication related to the SPIN dynamics package: https://doi.org/10.1016/j.jcp.2018.06.042

The formula is a standard spin hamiltonian that you can find in most statistical mechanics textbook. So the dimension of H (and J) is the same as energy. The units will depend on the settings of the LAMMPS units command.


Hello Wei and Germain!

Wei, Germain is perfectly right (and actually, you were right as well). J has units of energy, and in the expression you cite (the Heinsenberg Hamiltonian as implemented in the SPIN package), Si and Sj are dimensionless vectors. But as Germain said, I think you should find most of those definitions in the manuscript that presents the methodology.

Just one quick comment about the units: the SPIN package only works with metal units. My apologies for this limitation.

Hope this helps!
All the best,