Antisymmetrized geminal power (AGP) wave-functions in finite and extended systems

In extended systems, where the effective electron-electron interaction is attractive under certain conditions, the formation of bound electron pairs has utmost importance. The Cooper pairs are, in turn, related to the geminal appearing in the BCS wave function in a simple way. For finite systems the relationship is not so apparent. An approximate one can be obtained, and used, for analyzing AGP wave functions for atomic and molecular systems. This allows for a detailed picture of the correlation effects involved, their basis-set dependence as well as of the variation with internuclear distance. The picture is more detailed than the one displayed by the natural-orbital occupation numbers and is a unique feature of AGP wave functions.