https://github.com/gromacs/gromacs/blob/master/share/top/oplsaa.ff/ffbonded.itp#L1669 has an issue. For opls style torsions, they must sum to 0. Looking at the cited Amber forcefield, the parameter seems to be pulled from https://github.com/openmm/openmmforcefields/blob/ca9319f15d0c5d8565853c0f525b54373cfffae1/amber/ffxml/ff94.xml#L5242. Applying the [gromacs conversion](https://manual.gromacs.org/current/reference-manual/functions/bonded-interactions.html#proper-dihedrals-ryckaert-bellemans-function) to RB torsion parameters from the opls style, the correct parameters should be [-8.9956 3.138 10.0416 -4.184 0.0 0.0]. See https://github.com/mosdef-hub/mbuild/pull/916#issuecomment-859073283 for more information.