Simple systems with intramolecular hydrogen bonds are analysed here with the use of DFT and ab initio methods; B3LYP/6-311++G** and MP2/6-311++G** calculations have been performed. OH, SH, NH2, and CH3 groups are chosen as proton donors and N, O, and S atoms as accepting centres. The results of calculations show that OH bond is the strongest proton donor and the nitrogen centre is the strongest acceptor. Additionally the Bader theory is applied here showing that characteristics of the bond critical points and the ring critical points are useful parameters to estimate the strength of intramolecular hydrogen bonding. (C) 2003 Elsevier B.V. All rights reserved.