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ABSTRACT:If in atoms only one type of the electron affinity (EA) can be defined, in molecules there are three types of EAs: the vertical electron affinity (VEA), adiabatic electron affinity (AEA), and vertical electron detachment energy (VEDE). These three types of EAs for beryllium, magnesium, and calcium dimers and trimers are calculated at the allelectron MP4(SDTQ) level employing the Dunning-type basis sets. All obtained EAs satisfy the following inequality VEDE Ͼ AEA Ͼ VEA and are quite large to be observed in experiment, especially in the trimer case: VEDE (Be 3 Ϫ ) ϭ 1.63 eV, VEDE (Mg 3 Ϫ ) ϭ 0.72 eV, and VEDE (Ca 3 Ϫ ) ϭ 0.95 eV. The decomposition of VEDE into physical components (Koopmans, relaxation, and correlation) and the atomic orbital population analysis (at the NBO level) are used to elucidate the nature of the outer electron binding in studied anions.