Can not use OPC/3 water with ff14SB

[jthorton]

We have found that it is not possible to parameterise a system using opc/3 and ff14SB (see OpenFreeEnergy/openfe#869). I think this is due to a clash in residue names, ff14SB contains a residue named U which clashes with an ion parameter in the opc ions file. I noticed that other water force fields have this ion residue named U4+ see tip3p, tip4pfb. Would it be possible to fix the opc and opc3 files to match the other water models?

[epretti]

Thanks for finding this. It appears that this inconsistency is not limited just to the files you pointed out, but in fact all of the water/ions XML files in amber use U4+ and all of the ions files in amber/ions use U.

I have been working on updating openmmforcefields' CHARMM force field files to the latest version of those parameters. I plan to do Amber next, so I will take a look at this then, see if there are any other similar inconsistencies or clashes, and fix them. In the meantime as a temporary workaround to get something running, one should be able to copy the XML files and rename the offending residue(s).