The first test set consists of a series of zwitterionic tripeptides using charged glycine terminals. However, this type of tripeptide cannot be used to test the parameters of the disulfide-bonded cysteine (CYX). Instead, a dimer made from two tripeptide units was manually constructed using the Maestro program. Following this, a conformational search was carried out with the ConfGen module, keeping the 32 lowest energy conformers. On each of these conformers, single point calculations were carried out at B3LYP 53 /6-31G** 54 level of theory using the Jaguar program. The lowest energy conformer was retained for use in the following calculations. Strictly speaking, this molecule consists of six amino acid residues and is therefore not a tripeptide, but we include it in the tests below to be able to address the performance of the CYX parameters. For the remaining tripeptides, geometries were taken from FragBuilder. Geometry optimizations were carried out on all of the tripeptides at PBE0/def2-TZVP 57 level of theory using the ORCA program. The RIJ-COSX approximation was employed with the def2/J 59 auxiliary basis set. Water solvent was included using the C-PCM in order to avoid internal hydrogen transfers.