Abstract: The method of natural bond orbitals is used to investigate the aromaticity of cyclic conjugated molecules.The pseudo-delocalization energy of the natural bond orbitals is defined and adopted as a criterion for measuring the aromaticity of molecules.The pseudo-delocalization energies of the natural bond orbitals of more than twenty organic or inorganic molecules were evaluated by INDOor DV-Xα method and a natural localization program respectively.It is shown that the aromaticity order of these molecules evaluated on basis of the pseudo-delocalization energies is essentially in accordance with those reported in the references, which shows the reasonableness and usableness of the aromaticity criterion proposed in this paper.

Key words: Aromaticity, Pseudo-delocalization energy, Natural bond orbitals