My question regards the meaning of the field "potential" in the case of TNG300-3-dark where I only have dark matter particles. You describe it to be the "Gravitational potential energy" and you give its units (km/s)^2 / a. Those units, though, pertain to the potential energy per unit mass which is the same as the gravitational potential difference per unit mass (or just the gravitational potential if the reference is at infinity and the potential is zero there). So my first question is: do I just need to multiply the field "potential" with the mass of each particle (which is just one number) to get its potential energy? The second question has to do with your note that the potential is not available in "dark matter only runs". Is that still the case? Because I have started to use it.
Thank you in advance
The units of (km/s)^2/a are correct for the Potential field, so if you want a "potential energy" which has in it mass units, you should as you say multiply by the DM particle mass. You can check the code itself if you need any more details, e.g. how Potential is handled in accel.c.
This comment about "not available in DM only runs" I think is an old remnant for original Illustris, and does not apply to TNG - I'll remove it, thanks!.
the lack of potentials for DM only runs still seems to be true for TNG100-3-Dark
"Exception: Particle type  does not have field [Potential]"