Extended bond-like orbitals are included in molecular-like orbitals, beside the localized atomic-like orbitals, and an effective hamiltoni-an governing the chemical bond is derived; the derivation is based upon the difference between the atomic-and bond-scale lengths. The effective hamiltonian introduces the interaction between the electrons participating in chemical bond and their electronic "holes" in the atomic-like orbitals. A fractional occupancy is established for the bond-like orbitals, which is specific to the chemical bond
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