The central atom is indexed from 1, i.e. 3 indicates the third atom.
| Package | Improper type | Central atom | Links and notes |
|---|---|---|---|
| AMBER | Periodic | 3 (or 2) | Torsions can be reversed for 1-4 torsions. MDAnalysis/mdanalysis#2386 openmm/openmm#220 (comment) |
| ParmEd | Periodic | 3 | MDAnalysis/mdanalysis#2386 |
| ParmEd | Harmonic | 1 | MDAnalysis/mdanalysis#2386 |
| Gromacs | Periodic | 1 | MDAnalysis/mdanalysis#2386 |
| Gromacs | Harmonic | 1 | MDAnalysis/mdanalysis#2386 |
| OpenFF | Smirnoff (periodic) | 2 | openforcefield/openff-toolkit#746 (comment) |
| OpenFF | Amber | 3 | openforcefield/openff-toolkit#746 (comment) |
| CHARMM | all? | 1 | michellab/Sire#193 (comment) |
| MDAnalysis | all | any | MDAnalysis does not check connectivity. (However, calculating values of improper torsions assumes the central atom is first.) |
| NAMD | all? | 1 | Follows CHARMM |
| OpenMM | all | 1 | http://docs.openmm.org/7.2.0/userguide/application.html#periodictorsionforce |