The resonant Auger electron spectrum for ethene has been calculated with an ab initio approach using configuration-interaction energies and wave functions for the intermediate core-excited and final states. The transition rates were determined by the "one-center approximation." The role of vibrational relaxation on the line shapes was described by a moment method which considers the case of symmetric core holes and their localization due to the vibrational relaxation of the core-excited state. The core hole localization is investigated in some detail and is found to be extremely efficient in the C 1s(-1)pi* excited state of ethene. Another property of the core-excited state is found to be the polarization of the valence electron density toward the core hole. We demonstrate this by using three different symmetric configuration interaction representations and one nonsymmetric Hartree-Fock representation for this state. A modified improved virtual orbitals method is described and employed to obtain virtual orbitals which give a compact description of this effect. The theoretical spectra obtained in this way are compared with a measured spectrum and assignment of the structures in the spectrum to electronic configurations is made. We find strong configuration mixing in the higher excited final states which is evidence for the breakdown of the one-particle picture. (C) 2000 American Institute of Physics. [S0021-9606(00)31213-2].