The calculation of energies and lifetimes of metastable molecules requires the treatment of both the continuum and correlation effects. We describe the complex absorbing potential approach incorporated within a configuration-interaction framework. The absorbing potential method allows a very efficient solution of the continuum problem, making possible a detailed study of the correlation effects that turn out to be surprisingly strong. The famous N − 2 2 g resonance is studied as a test case and much attention is paid to an internally balanced treatment of the metastable state. Our findings are rationalized within a simple model that is then used to understand the results of various previous studies.

In this work we focus on the binding of excess electrons to water clusters, a problem for which dispersion interactions, which originate from long-range correlation effects, are especially important. Two different model potential approaches, one using quantum Drude oscillators and the other using polarization potentials, are investigated for describing the long-range correlation effects between the weakly bound excess electron and the more tightly bound electrons of the monomers. We show that these two approaches are related in that the polarization potential models can be derived from the quantum Drude model approach by use of an adiabatic separation between the excess electron and the Drude oscillators. The model potential approaches are applied to clusters containing up to 45 water monomers. Where possible, comparison is made with the results of ab initio electronic structure calculations. Overall, the polarization potential approach is found to give electron binding energies in good agreement with those from the Drude model and ab initio calculations, with the greatest discrepancies being found for "cavity-bound" anion states.

Nitromethane is a prototypical example for a molecule that can bind an extra electron in two fundamentally different ways forming dipole-bound as well as valence anions. The classification of the electronic states as dipole-bound or valence does in fact suggest a diabatic viewpoint, and we investigate the coupling between these two electronic states of the nitromethane anion. The coupling element W is extracted from a cut through the two lowest adiabatic potential energy surfaces by fitting of a simple avoided crossing model potential, that is, W is effectively approximated as half the smallest splitting. High level ab initio calculations are performed to compute the two states along the cut. We discuss in particular how a balance between the two very different electronic states can be achieved, and how the temporary nature of the valence anion in a large region of the relevant nuclear coordinate space can be taken into account. The autodetachment lifetime following vertical electron attachment to the neutral is computed, but the calculation of the temporary anion state turns out to be too expensive for a study of the two adiabatic surfaces, and consequently, the second adiabatic state is only included at geometries where it lies below the neutral potential energy surfaces. We find a coupling matrix element of 30 meV. On the one hand, this value is much smaller than the vertical excitation energies underlining the need for a diabatic picture. On the other hand, this value suggests rapid transitions on a mass spectrometric timescale substantiating the notion that the dipole bound state provides an efficient doorway for attachment to the valence state.

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