Definition of a multicenter bond index

The tensor character of the first-order density matrix leads to the definition of an MO multicenter bond index for closed-shell systems. It is here applied to three-center bonds. Satisfactory results are obtained for compounds involving secondary bonds, strong and normal hydrogen bonds; the index for the peptide bond is found to be similar to that of strong hydrogen bonds.On leave of absence from Inst. de Física, UFBA, 40000 Salvador, BA, Brasil.