Composite particles in quantum chemistry: From two-electron bonds to cold atoms

We review the problem of forming composite particles from elementary electrons. Simple pairs are treated first, followed by linear combinations including geminal type wave functions. The antisymmetrized product of strongly orthogonal geminals (APSG) theory can also be developed in this language, and the use of APSG as a reference state is discussed. The theory forming composite particles from more than two electrons is less elaborated, although this idea is routinely used in approximate models in the current literature. (c) 2012 Wiley Periodicals, Inc.