Accurate evaluation of coupled-cluster ionization potentials and electron affinities via excitation energy calculations

An alternative approach for obtaining accurate vertical ionization potentials (VIPs) and electron affinities (VEAs) via coupled-cluster excitation energy calculations is proposed. The concept allows a coherent handling of all ionic states, including ionizations from lower valence orbitals and attachments to higher-lying virtual ones. The use of existing, widespread quantum chemistry codes with minimal modifications makes the application of well-established wave function models possible, in full consistency with the treatment of charge transfer excitations. Among them, the spin-component scaled forms of the CC2 and ADC(2) methods are potent approaches, especially the scaled opposite-spin variants whose efficient implementations allow the handling of larger systems. The performance of several models is evaluated via benchmark calculations on various sets from previous works, containing small and medium-sized systems, including nucleobases. It is shown that with the most effective scaled approximate methods the accuracy of EOM-CCSD is achievable at a fraction of the computational cost, also outperforming many common electron propagator approaches.