OAHÁ Á ÁS intramolecular hydrogen bond (IHB) in thiomalonaldehyde and its various derivatives were studied at MP2/6-311++G (d, p) level of theory and corresponding IHB energies were obtained using the related rotamers and open-close methods. In addition, the topological properties of electron density distribution and charge transfer energy associated with intramolecular hydrogen bond were gained by quantum theory of atoms in molecules and natural bond orbital theory, respectively. The results of this study showed that the related rotamers hydrogen bond energy correlates well with geometrical parameters such as OÁ Á ÁS and HÁ Á ÁS distances, topological parameters such as electron density, its Laplacian and electronic potential energy density, V(r cp ), at hydrogen bond critical point and ring critical point as well as charge transfer energies in chelated ring. Surprisingly, it was found that the open-close hydrogen bond energies can not represent similar linear or even polynomial correlations with these parameters. Consequently, the open-close method is not suitable for estimation of the IHB energy and erroneous, while the related rotamers method is convenient for estimation of the IHB energy.