Hey, for your PolyIL + LiTFSI sim in GROMACS, the force field probably misses Li+ non-bonded params because it's custom for the polymer/IL. Add them to ffnonbonded.itp using standard OPLS-compatible values like Joung-Cheatham (compatible with OPLS):

For Li+ (assuming combination rule 3, sigma in nm, epsilon in kJ/mol):

[ atomtypes ]

; name atnum mass charge ptype sigma epsilon

LI 3 6.941 0.0 A 0.1894 0.0766 ; for TIP3P-like, adjust if needed for your setup

Then in ions.itp or your topology, define the Li ion with charge +1.0.

If your FF uses different combo rules or units, double-check. Lit search for "OPLS-AA Li+ parameters" if these don't fit.

GAFF is mainly used for standard organic molecules, you might take a look here:

DOI: 10.1021/acs.jcim.5b00674,

https://doi.org/10.1021/acs.jctc.4c01272

https://doi.org/10.1021/acs.jctc.4c00850

hope this help