eq:Psik eq:Psik with y R r describing the group R, which collects N R electrons in the state r, and A is the partial antisymmetrizer operator, which exchanges the electron coordinates x between two, or more, different groups. The group functions y R r are individually antisymmetric with respect to the electron exchange: y R r (...,x i ,...,x j ,...) = y R r (...,x j ,...,x i ,...)