CAP/SAC-CI method for calculating resonance states of metastable anions

The complex absorbing potential (CAP)/symmetry-adapted cluster-configuration interaction (SAC-CI) method has been developed to investigate resonance states of metastable anions. The method has been implemented in the projected scheme and applied to the π∗ resonance state of formaldehyde. The dependence on both valence and diffuse basis sets up to g-function, the number of SAC-CI states in the projection, and the effect of perturbation selection are examined. The potential energy curve and decay width are calculated in the C–O stretching coordinate, and the Franck–Condon factors for transitions from neutral to resonance state are evaluated to interpret the electron transmission (ET) spectrum.