Dear Lammps users,

I simulated a box of 200 of ionic liquid molecules in NVT ensemble using:

pair_style lj/cut/coul/long 11

kspace_style pppm 1.0e-4
when I use fix 3 all press/berendsen iso 1184 1184 100.0, it produces an “out of range atoms - cannot compute pppm”, but without this fix it is ok, but the pressure data is not well-fitted.
before the above-mentioned error there is a warning " improper problem, which below this warning for all four atoms it produces nan values".
I used topotools to produce data file, and now, I found out that one type of my molecules improper is missing. Is there any easy way to add that improper_type. Is it possible that this missing improper type would be the cause of my bad results with great Error percent compare to exp. data?

Also, I used opls for dihedral because all the dihedrals and impropers in the article that I used to extract them is in opls form, but Lammps do not have opls form for improper, so I used cvff ( because first and third and fourth coeff. of improper were zero) and convert the coeffs to cvff form. Is this wrong?

It’s hard to say why your MD run terminates with that error- it could be multiple things. For one thing, pay attention to units that are used in this simulation. See for details. For example, what units are used for the pressure in this simulation? I would have assumed that you are using “real” units, which means that you are trying to impose a high pressure on this system. Also, you need to make sure that your coefficients for bond/angle/dihedral/improper terms are consistent with the simulation units.