General-model-space perturbation theory: Excitation and ionization of N2

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Abstract
Vertical excitation and ionization energies are calculated by diagrammatic many‐body perturbation theory (MBPT) for 20 states of N2 and N+2 at R=2.068 bohr. Nondegenerate or degenerate MBPT with complete or incomplete model spaces is employed, depending on the molecular state. A variety of zero‐order Hamiltonians (H0) is tried. It is found that [2/1] Padé approximants depend little on the choice of H0, making them superior to straightforward summation to third order. They also provide good approximations to the CI limit for the basis (average error for all states 0.35 eV).