Abstract: Köster's method for separating the DFT energy of benzene into π and σ parts is used to investigate the nature of the π delocalization energy in a nonplanar molecule benzylideneaniline(NBA). Contrary to the classical viewpoints, the vertical resonance energy ΔE^V is always destabilizing, and it is most destabilizing at about θ=0° geometry at B3LYP/6-31G*, 6-311G*, 6-31G(2d), 6-311G(2d) levels. Besides, the one electron energy of the π system is more sensitive to π delocalization than that of the σ system. Furthermore, it is the electronic coupling between the π and σ electrons, rather than the π-π and σ-σ interaction, that plays a main role in determining Coulomb and the exchange-correlation energy effects associated with the π interaction between fragments.

Key words: Energy Separation of DFT, π-Electron delocalization, Benzylideneaniline