The purpose of the present paper is to describe a method,
in quantitative sense, of discussing the isolobal analogy of fragments
(or molecules) proposed by Hoffmann. The »Frontier Hybrid
Orbitals (FHO)« are used to represent the directional, fronti er
orbitals of the central atom, which have the same meaning as the
»lobes« involved in the concept of isolobal analogy. The calculatio
n of Frontier Hybrid Orbitals (FHO\u27s) is based on the natural hybrid orbitals which are the eigenvectors of density matrix blocks. The overlap integrals, bonding capabillties, and valence orbital energy levels of the FHO\u27s of fragment s are also calculated. The relationship between FHO and the isolobal analogy is discussed in this paper