Neigh_modify exclude dynamically

[tomasfbouvier]

I’m running a simulation for which I would like to dynamically exclude some atoms from the neighbour list building. It semms that when using a dynamic group the neig_modify command simply ignores it. I found another issue where this problem was commented but no solution was provided for this particular issue.

Neigh_modify exclude

Dear all, May I know if the neigh_modify exclude option will work correctly with dynamic groups? The documentation pages for ‘neigh_modify’ and ‘group’ do not mention any such restriction but my testing indicates otherwise. My objective is to simulate a moving wall boundary by turning off all pairwise interactions involving wall atoms located within a region whose extent changes with time. Best, Karthik DISCLAIMER The sender of this email is an alumnus of National University of Singapore (NU…

neigh_modify works perfectly fine with static groups and I checked that my dynamic groups are not empty

Example:

region          ss sphere 50 50 50 100
group           mobile_static region ss
group           mobile_dynamic dynamic all region ss every 1
neigh_modify every 1 exclude group mobile_dynamic

The command excludes properly for static but not for dynamic

[akohlmey]

When reporting unexpected behavior, please always report which LAMMPS version you are using and what platform you are running on.

@tomasfbouvier

The command excludes properly for static but not for dynamic

This command should create an error since there is only one group given, but you need two.

[akohlmey]

From looking at the source code that updates the group bitmasks for atoms, I can see that there could be an issue using dynamic groups for neighbor lists, since they are updated after the neighbor list build.

I can confirm that. We are looking into what would be a proper solution for this, but for the time being the only path would be to throw and error when dynamics groups are used for neighbor list exclusions.

[tomasfbouvier]

Yes indeed. I copied the command improperly. With the right command it does not work either. My lammps version is 27 Jun 2024

[vsorichetti]

Dear @tomasfbouvier , I only recently saw this post.

I’ve recently used this combination (neigh_modify exclude with a dynamic group) in some simulations I’ve run, and it seemed to behave as expected in my case (I was using the LAMMPS stable release of 02/08/2023).

If you remember what was the problem in your case, could you please share some more details?

I also see that in the current LAMMPS stable release (22/07/2025) the problem seems to be still not solved.

Thanks,

Valerio