A procedure for constructing a highly localized and symmetrical bond orbital basis set with the πsystems separated off from the σ frameworks has been developed. It is a four - step procedure: (1) over the opened - shell localized fragment molecular orbital (FMO) basis set [φk，φi，φj] where φi ∈ doubly occ. and vacant σFMOs, φj ∈πFMOs, and φk∈ singly occ. FMOs, the conditional RHF computations provide each of the FUL and DSI°electronic states of a molecule, such as norbornadiene with a set of the closed - shell FMOs; (2) the symmetrical MOs ,Φ°l' = Σakl'φk(k=1,2, …,Ns) which have delocalized over the whole molecule, in the DSI° substitutes for the unsymmetrical Φl= Σaklφk in FUL state, and those together with other two groups of the unsymmetrical FMOs, Φm= Σajmφj and Φn= Σ ainφi in the FUL state formed a closed - shell FMO basis set [Φn,Φm, Φ°l'] in which each of FOMs Φn and Φm is still localized on its corresponding fragment; (3) based on the basis set [Φn,Φm, Φ°l'], the conditional RHF computation for molecule is performed under the following constraint: all Fij=0.0 and Sij=0.0(i≠j, i∈fragment Q, and P≠Q) execpt for those between Φ°l'. It provides a molecule, such as norbornadiene, with a highly localized and symmetrical FMO basis set [ Φn', Φm', Φl']; (4) each of the FMOs Φn', Φm' and Φl' is concentrated on a specific atom or two neighboring atoms using the Perkin procedure at last, and it has correct orbital occupancy.

Key words: PERTURBATION MOLECULAR ORBITAL, MOLECULAR ORBITALS CALCULATIONS, LOCALIZATION