SO4 ion and the O atoms distance

How can I incorporate the special distance between oxygen atoms in the SO4 ion according to the specified force field, considering there are no bonds between them?

The distance values for the \textrm{SO}_4 geometry seem to be given in nm and not in Å.
Not sure if I would want to trust a paper with such obvious errors. Where were the reviewers?

Impossible to confidently say something specific about how to enter the geometry. Since the it mentions TIP4P/2005, which is a rigid model, I would guess that the sulfate is also rigid. This can be done with fix rigid or fix rigid/small or its (many) variants.

Hi Axel,
Good catch! I asked the author (J. Chem. Phys. 151, 134504 (2019)) about it! Yes that should be nm not A!
In this FF, the S atom has been considered as a dummy atom with no mass and the mass of the S atom is equally distributed to the O atoms!
I use the rigid/small for SO4 but after a few pico second simulations the pppm error occurs (Out of range atoms - cannot compute PPPM)!

lammps version: lammps-23Jun2022

That error can have a lot of reasons. Usually some mistake with creating the topology or geometry or some incorrect simulation settings.

This error has been reported many times and suggestions have been made almost as many times, so please have a look at the archives.

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