Full geometry optimizations were carried out for 2-pyrrolidinone (2-Py) and its dimer structures at the DFT (B3LYP) and HF levels in the gas phase and in solution. Additionally, single-point MP2/6-31G** calculations were performed on the HF/6-31G** optimized geometries. The self-association of 2-Py in 1,4-dioxane (epsilon =2.21) and in water (epsilon =78.54) were studied using the self-consistent reaction field (SCRF) theory. The calculated relative free energies (DeltaG) for the dimerization processes predict the cyclic dimer (CD) to be more stable than the open dimer (OD) at all levels of theory in the gas phase and in solution. The results show that, at least for the properties of the hydrogen-bonded systems studied in this work, the B3LYP/6-31G** calculations give better results than the HF and MP2 calculations with the same basis set. The data obtained from this study were compared with the corresponding experimental results when available.