Valence states of the cyano radical Feynman's way

Scaled Feynman diagrams are proposed as perturbative corrections to the total energy. The proposal is tested by calculating potential energy curves for the valence states of the cyano radical. Unique features of the valence state are part of the starting approximation. The latter is computed with a standard quantum chemical method. For a particular method, the scale is independent of the valence state. At each bond distance, one set of quasiparticles and Feynman diagrams is used for all valence states. Scaling will be useful whenever perturbative corrections can be expressed with Feynman diagrams. In the cyano radical, competition for the odd electron between the σ and π orbitals produces a variety of valence states. Many are quasidegenerate. The 16 states investigated here are doublets and quartets with triple bonds, double bonds, ‘resonance’ bonds, bonds intermediate to double and triple bonds, and bonds intermediate to single and double bonds. Energy curves and spectroscopic constants are reported for the lowest lying quartets and two or more lowest doublets of Σ ± , Π, and Δ symmetries, and also 1 2 Φ symmetry. Inner and outer minima in the energy curve for 3 2 Π are assigned to H and G , respectively. A more accurate assignment of a 4 Σ + is made. Relative to experiment, maximum (average) errors in bond distance, frequency, and the adiabatic excitation energy are 1.6 pm (1 pm), 60 cm −1 (50 cm −1 ), and 19 kJ/mol (5 kJ/mol), respectively. The error in the ground state dissociation energy is 11 kJ/mol. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002