In contrast to bound anions, temporary anions lie energetically above their parent neutrals and can therefore decay by electron autodetachment. Therefore, temporary anions are part of the electron scattering continuum, and they are characterized by their energy above the neutral, the so-called resonance energy, and by their width, which is inversely proportional to their lifetime. Despite much recent progress, computing both the energy and lifetime of a temporary anion remains one of the notorious challenges of quantum chemistry, because it combines an electron-scattering with an electron-correlation problem. In other words, the wavefunction of a temporary anion exhibits, on the one hand, bound-anion-like character, where all N+1 electrons occupy valence electrons, and, on the other hand, neutral-plus-free-electron character, where N electrons occupy valence orbitals, and one electron occupies a free electron state. Electron correlation turns out to be crucial regarding the exact mixing of these two aspects, and in particular the computed lifetimes are very sensitive to the computational level.<br/><br/>Any method addressing this challenge must be a combination of an approach to address the continuum nature of the wavefunction and an electronic structure method. Here the temporary state is separated from the continuum using a complex absorbing potential, and this continuum method is combined with the symmetry-adapted-cluster configuration-interaction (SAC-CI) method for electron attachment. The SAC-CI method is closely related to the equation-of-motion coupled-cluster method for electron attachment: Both are highly accurate methods for computing electron affinities; both are direct, multi-state methods; the only difference are some approximations in the cluster operators that make SAC-CI somewhat more approximate but also more efficient. CAP/SAC-CI results are presented for molecules containing up to 10 non-hydrogen atoms, and for some of the molecules, three temporary states in the same symmetry have been characterized.

Studies of medium-size temporary anions with complex absorbing potentials and the SAC-CI method

251st National Meeting (2016)

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