2-Pyrrolidinone is a model compound for peptides, proteins and antibiotics. We performed full geometry optimizations for -CN, -NH2, -CH3 and -C(CH3)(3) derivatives of 2-pyrrolidinone and their cyclic and open dimer structures at the HF/6-31G** and B3LYP/6-31G** levels in the gas phase and in water. Additionally, single-point MP2/6-31G** calculations were performed on the HF/6-31G** optimized geometries. The influence of the solvent was examined using the self-consistent reaction field calculations. The effect of the attached groups on the structural properties and the dimerization processes of the 2-pyrrolidinone were discussed. (C) 2004 Elsevier B.V. All rights reserved.