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 pi* 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. (C) 2012 Elsevier B.V. All rights reserved.