Hook vs History

Dear Lammps Developers,

I am using gran/hook to simulate frictional particles.


pair_style gran/hooke 1 1 1 1 0.5 1


pair_style gran/hooke/history 1 1 1 1 0.5 1

provide tangential forces and make particles spin along z direction in 2d.

However, according to lammps documentation:

" The shear force is a “history” effect that accounts for the tangential displacement between the particles for the duration of the time they are in contact. This term is included in pair styles hooke/history and hertz/history, but is not included in pair style hooke"

Therefore, gran/hooke should not give rise to tangential forces !

What is really the difference between




In advance, I appreciate your kind reply.


Read the documentation more carefully. In particular, look at the entire expression with the “t” subscript.

I’m sure that if you spend a little bit of time thinking about what variables are in the force model, you’ll come to understand why the term is called a “history” force. And why it might be more expensive to compute - and hence why there is an option to shut it off.

Alternatively, set-up a binary particle system (two particles colliding) with prescribed motion (with different components turned off and on) and you should be able to test your theories of what is a history term and what is a tangential component. Only takes a little bit of effort and you won’t have to question it again.