Abstract: A simple non-variational configuration-interaction theory is presented for the divalent atomic system with two electrons outside a closed shell core, such as an alkaline earth atom and its corresponding isoelectronic sequence. The theory is used for exact solution of the multiconfiguration state wave function and its corresponding energy eigenvalue in the system, and for quantitative analysis of configuration interaction which reflects the exchange and correlative interaction between electron-electron. Discussion focuses on the use of a nearly complete finite basis set construeted from B-spline which is capable of taking into account effectively the continum contribution from positive-energy one-electron orbitals confined in a finite volume. Numerical example is for Be atom of the divalent atomic system and the obtained result conforms excellently with experiment.

Key words: divalent atom, configuration-interaction, multiconfiguraction state wave function