Feshbach resonances in electron-molecule scattering by the complex multiconfiguration SCF and configuration interaction procedures: The ¹Σ_g⁺ autoionizing states of H₂

The complex resonance energies (positions and widths) and wave functions for the lowest ¹Σ_g⁺ doubly excited autoionizing states of H₂ are directly computed by using the multiconfiguration self-consistent field (MCSCF) method and the configuration interaction (CI) method within the context of the complex basis function technique. These autoionizing states are Feshbach resonances (as opposed to shape resonances), and single-configuration self-consistent field calculations provide no information about the lifetimes of such states. All of these methods rely on the existence of a complex variational principle for complex resonance energies. It is shown that by using a small orbital space the MCSCF method can give essentially the same complex energies as the full CI method. Numerical results are in good agreement with previous theoretical results, especially with the optical potential calculation employing a diffuse basis set by Schneider and Collins [Phys. Rev. A 28, 166 (1983)], indicating that poor agreement with previous complex coordinate results by Moiseyev and Corcoran [Phys. Rev. A 20, 814 (1979)] is attributable to their small basis set. A detailed study of basis set and correlation effects on the complex energies is also presented.