In the overwhelming majority of cases the resonances observed in electron transmission spectroscopy can be explained in terms of temporary anions. These anion states are formed when a free electron with a positive energy attaches temporarily to a neutral molecule, and are consequently characterized by two quantities, an energy above the neutral and a finite lifetime. Investigating temporary anions computationally is challenging, owing to the so called combined continuum-correlation problem. On the one hand, their positive energies makes temporary anions part of the electron-scattering continuum, which forestalls the use of standard bound-state methods. On the other hand, there are typically large electron correlation effects between the �excess� and the valence electrons, which calls for high level ab initio methods. It is convenient to classify theoretical methods for temporary anions according to how the electron-scattering continuum is treated: (1) not at all; empirical scaling methods, the lowest rung. (2) through an artificial stabilizing potential and extrapolation to zero stabilization; extrapolation methods. (3) through absorbing or box-type boundary conditions; complex scaling, complex absorbing potential, and stabilization methods. And (4), proper scattering boundary conditions; the highest rung. Needless to add that there is a stark trade-off with treating electron correlation while climbing the ladder. Here the performance of two implementations of the extrapolation method are investigated. The first approach uses scaled nuclear charges, which is easy to implement, but leads to a long-range stabilizing potential. The second approach uses a short-range stabilizing potential.

Extrapolation methods for predicting the energy and lifetime of a temporary anion

248th National Meeting (2014)

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