Perturbation theory for nonsymmetric matrices is discussed for the GF matri
x for molecular vibrations. As a simple extension of early results, two app
roaches are given: one a direct diagonalization of the nonsymmetric matrix,
the other a presymmetrization. Presymmetrization of the GF matrix, well kn
own for DeltaF, is also described here for DeltaG. It permits the use of st
andard perturbation theory for symmetric matrices. Application of the secon
d-order expression for the DeltaG case to the determination of the frequenc
ies of many ozone isotopomers is given elsewhere.