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We propose a simple method for calculation of low-lying shape electronic resonances of polyatomic molecules. The method introduces a perturbation potential and requires only routine bound-state type calculations in the real domain of energies. Such a calculation is accessible by most of the free or commercial quantum chemistry software. The presented method is based on the analytical continuation in a coupling constant model, but unlike its previous variants, we experience a very stable and robust behavior for higher-order extrapolation functions. Moreover, the present approach is independent of the correlation treatment used in quantum many-electron computations and therefore we are able to apply Coupled Clusters (CCSD-T) level of the correlation model. We demonstrate these properties on determination of the resonance position and width of the (2)Πu temporary negative ion state of diacetylene using CCSD-T level of theory.

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The theory of Fano profiles currently presented in the framework of scattering theory can also be investigated from model Hamiltonians projected in the basis of discrete states. It is shown that the wave operator approach of quantum dynamics applied to these models simultaneously provides the lineshapes and the dynamics of the quasi-bound states of interest. An analytical expression which generalizes Fano profiles is presented.