Gas adsorption: fix rigid/npt/small + fix nvt

LAMMPS LAMMPS General Discussion
1

Hello!

I try to simulate the adsorption of CO molecules (gas) on a Pd nanoparticle. Molecules are set as rigid bodies. I’ve received a successful experience using fix nvt/rigid/small for CO and fix nvt for Pd np. But now I want to control gas pressure. But when I use fix npt/rigid/small for CO and fix nvt for Pd np, the simulation always ends up with error “Non-numeric atom coords - simulation unstable“. I can’t add energy minimization (advised for this error), because “fix rigid is not supported during minimization“

My input (the same was used for fix nvt/rigid/small + fix nvt) :

(lammps_2025_07_22)

ads-md.in (4.1 KB)

sphere_27A.data (44.0 KB)

CO_TIP4P.txt (371 Bytes)

I have no idea how to fix it. Please help.

P.S. Here, CO has a third dummy atom (named N for better treatment) with no mass. It represents the electron density between C and O to improve electrostatic distribution.

2

Using minimization is just one possible option. Please note that the paragraph first mentions too aggressive simulation settings or parameters.

Your simulation has a timestep of 1fs. That is absolutely far too large for both using ReaxFF and small rigid bodies. So the first thing to try is to reduce the timestep.

The second thing is that it is usually a bad idea to use fix npt at the beginning of an equilibration if your system has a very high potential energy. The system can “explode” and it will take a very long time for it to return to a sane status, if at all.

Another option is to keep only part of the system mobile, e.g. use fix setforce to set the forces on the atoms of the rigid bodies to zero, so they won’t move during minimization and then minimize only the nano particle.

3

Thanks, now I see