Spin package

Hello everyone. Does anyone know why the magnetic force calculated by LAMMPS is in terms of Rad/THz? Using the spin package in LAMMPS, I want to work on the separation of gases (paramagnetic particles) and I have questions about the performance of LAMMPS. Can anyone help me?
if anybody can help please let me know.

That is the choice of the developer contributing the package.

What questions?

Why magnetic force is given in terms of rad / THz. How can it be interpreted? And why does LAMMPS not add it to the total forces acting on the particle?
and how can i see the rotation of spin toward the external magnetic field in NVT ansamble?
And how can i define a magnetic field varied with position and calculate the magnetic quantities with LAMMPS commands?

How does the vacuum permeability calculate in metal unit?

These are all questions for the person that developed and contributed the SPIN package to LAMMPS. If you cannot find the answers in the manual or the referenced publications, you have to contact him directly since I don’t think he’ll see them here.

unfortunately i could not find the answers in the manual or the referenced publications.
thank you

I have the impression that you have not read those with the right attitude. You are not likely to find direct answers to your questions, but rather you need to first learn what the package and its method does and how it is implemented and then you will gain insight that will allow you to answer your questions mostly on your own. Or you will no longer see the need to ask them.

Instead you seem to have made up your mind about what the package does and should be capable of based on your project and your expectations and that limits your ability to find answers.
The fact that you had restated your first question without acknowledging my response supports my assessment.

You are right.
But you said it is the choice of the developer contributing the package, but force has a certain meaning in physics, and my main problem is understanding the concept that comes from this selected unit and then I restated my question.
it seems this force is really different from other kind of mechanical forces that apply to particles and cause to move them.
i guess this force is a measure of rotation of spin. especially because its value have not been added to total force.
i studied the manuals but i could not find anything about that.

To understand the method you also need to study the publications explaining it. The manual is like a driver’s manual of a car: it tells you where the buttons are and what settings are available, but it does not explain how to drive or how a car works.

Hello @Somayeh !

My name is Julien Tranchida. My apologies for the late reply, I just registered on the forum and was mainly following the mailing list before.

The term in rad/THz is a precession vector, not an actual force. It corresponds to the “omega” vector in the LLG equation of each spin. See here for an example:

That being said, each magnetic pair interaction (for example pair spin/exchange) will also generate a force on the atoms, in eV/Ang. Overall, this is not really my choice, as this is all pretty standard in the spin - lattice literature.

Hope this helps! Let me know if you have additional questions.
All the best,

hello Julien
thanks for your reply and guidance.
According to the manual, if there is a dipole-diople magnetic interaction between particles, a mechanical force is created between particle i and particle j (between pairs of particles).
So why, when I used the pair_style spin / dipole / cut command to simulate the motion of particles under dipole-dipole interaction, the amount of force exerted on the particles is zero and the particles do not move? (There is only dipole-dipole interaction in the system and I have no other force field.)
Using the loop command, I wrote the algorithm for calculating this mechanical force with the formula provided in the Lammps manual, and this force is not zero.
Do I have to calculate this force this way (with an algorithm) and then apply this calculated force to the particles with the addforce command? This way I can see the motion of the particles.
In short, how can I see the motion of particles that are only affected by the magnetic dipole-dipole interaction?
Thank you very much for your help

Hello @Somayeh ,

I just ran some tests (using the “spin dipole cut” example in the input deck, and keeping only the spin/dipole/cut contribution). I do seem to see some mechanical forces, but they are just very small.
But for example, if you increase the norm of the spins you declare in your system, and you dump the forces in a file with a dump command, you should see them growing.

It is difficult for me to tell you why your script is not recovering the same mechanical forces as Lammps. There could be a lot of reasons: a small error in my implementation of the dipole-dipole term in Lammps is of course an option, unit inconsistencies is another one, … and a lot of other things.