It is very useful for understanding the chemical bonds in molecules to evaluate the electronic delocalization energy between different orbitals (for example, p-π → d-π and p-π → σ~*). The electronic delocalization energy could be obtained by performing HF-SCF calculations with and without some designated orbitals. Because of the interaction of orbitals the calculated delocalization energy is different if the order in which the orbitals are deleted is different. A progressive orbital deletion procedure is proposed to overcome this difficulty. The dominant characteristic of the procedure is that the Coulomb integral of a chosen orbital is shifted progressively by a one- electron operator. While the Coulomb integral increases step by step, the relevant orbital is selected from the molecular wavefunction step by step. In this way, the interaction between orbitals is decreased and the accurate delocalization energy can be obtained. The molecule of H_3PO is discussed as an example.

Key words: AB INITIO CALCULATION, CHEMICAL BONDS, MOLECULAR ORBITAL THEORY