The system density operator is expanded in powers of an arbitrary perturbation which may be electronic correlation and an efficient formalism is obtained for evaluating the matrix elements required to perform molecular property calculations. A generalized Brillouin condition is derived and used to define a Fock potential. The problem of assigning the poles of the polarization propagator to transitions of pure space and spin symmetry is discussed and the polarization propagator is expanded through second order in correlation for a spatially nondegenerate doublet state. Preliminary results for the Li(2S) and BeH(2Sgr;) systems indicate that agreement with experimental excitation energies is as favorable for open shell systems as for closed shells.
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