Structure Optimization of Temporary Anions.

We determine equilibrium structures, adiabatic electron affinities, and resonance widths of various temporary anions to benchmark the complex absorbing potential equation-of-motion coupled-cluster (CAP-EOM-CC) method. The second-order approximation to CAP-EOM-CC with singles and doubles (SD) excitations is found to yield slightly lower resonance positions and widths than full CAP-EOM-CCSD. The basis set dependence of adiabatic resonance positions and widths is similar to that of the vertical quantities. We demonstrate the usefulness of structure optimizations of temporary anions by two examples. For the anions of acrylonitrile and methacrylonitrile, we observe good agreement for the adiabatic electron affinities and structural changes between our theoretical results and two-dimensional electron-energy loss spectra. For the unsaturated hydrocarbons ethylene, 1,3-butadiene, and cis- and trans-1,3,5-hexatriene, the agreement between theory and electron transmission spectroscopy is good for the lower-lying π* resonances, while our results for the 3π* resonance of trans-hexatriene suggest a shortcoming of the method or reinterpretation of the corresponding electron transmission spectrum. The experimentally determined difference between the electron affinities of the 2π* resonances of the cis isomer and the trans isomer of hexatriene are reproduced well by CAP-EOM-EA-CCSD and CAP-EOM-EA-CCSD(2).