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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The electronic interaction between dipole-bound and valence anions of uracil and chlorouracil is investigated. In general, dipole-bound and valence states of an anion show very different electronic structures and the extra electron occupies completely different regions of space. Here, the coupling strength between the different attachment states is computed by fitting a simple diabatic model potential to a cut through the two adiabatic surfaces of the anions obtained from ab initio calculations. During these calculations, electron affinities of uracil and chlorouracil as well as resonance energies associated with vertical attachment into the valence orbitals of these molecules are obtained, and our results are compared with the available experimental and theoretical data. The estimated electronic coupling governs the intramolecular electron transfer from dipolebound to valence orbitals, and the associated transfer rate has implications for the mechanism of electron attachment and electron-induced bond cleavage of uracil and 5-chlorouracil with "zero-energy" electrons.

Complex absorbing potentials (CAPs) are imaginary potentials that are added to a Hamiltonian to change the boundary conditions of the problem from scattering to square-integrable. In other words, with a CAP, standard bound-state methods can be used in problems involving unbound states such as identifying resonance states and predicting their energies and lifetimes. Although in wave packet dynamics, many CAP forms are used, in electronic structure theory, the so-called box-CAP is used almost exclusively, because of the ease of evaluating its integrals in a Gaussian basis set. However, the box-CAP does has certain disadvantages. First, it will, e.g., break the symmetry of Cnv molecules if n is odd and the main axis is placed along the z-axis by the "standard orientation" of the electronic structure code. Second, it provides a CAP starting at the smallest box around the entire molecular system. For larger molecules or clusters, which do not fill the space efficiently, that implies that much "dead space" within the molecule will be left, where there is neither a CAP nor a sufficient description with basis functions. Here, two new CAP forms are introduced and systematically explored: first, a Voronoi-CAP (that is, a CAP defined in each atom's Voronoi cell), and second, a smooth Voronoi-CAP (which is similar to the Voronoi-CAP; however, the noncontinuously differentiable behavior at the surfaces between the Voronoi cells is smoothed out). Both have isosurfaces that are similar to the cavities used in solvation modeling. An obvious disadvantage of these two CAPs is that the integrals cannot be obtained analytically, but must be computed numerically. However, Voronoi-CAPs share the advantage of having the same symmetry as the molecular system, and, more importantly, considerably facilitate the treatment of larger molecules with asymmetric side chains and of molecular clusters.

In continuation of Paper I of this work we describe a practical application of the combination of complex absorbing potentials (CAPs) with Green’s functions. We use a new approach for calculation of energies and lifetimes of temporary anions, which emerge, e.g., from elastic scattering of electrons from closed-shell targets. This new method is able to treat the continuum and correlation effects simultaneously and reduces the problem to the diagonalization of a number of relatively small, complex symmetric matrices. The efficiency of the proposed method is demonstrated and its dependence on basis set and parameters characterizing the CAP is investigated using the Πg2 resonance state of N2− as an example. We also present the first correlated ab initio calculation of energies and lifetimes of resonances in elastic electron scattering from the organic molecule chlorobenzene. Our results for both examples are in good agreement with existing experimental values and other theoretical calculations. Possible future developments are discussed.

The interatomic Coulombic decay (ICD) in the Ne dimer is discussed in view of the recent experimental results. The ICD electron spectrum and the kinetic energy release of the Ne+ fragments resulting after Coulomb explosion of Ne2 (2+) are computed and compared to the measured ones. A very good agreement is found, confirming the dynamics predicted for this decay mechanism. The effect of the temperature on the electron spectrum is briefly investigated.