Singlet geminal wavefunctions

Wavefunction forms based on products of electron pairs are usually constructed as closed-shell singlets, which is insufficient when the molecular state has a nonzero spin or when the chemistry is determined by d- or f-$$f-$$electrons. A set of two-electron forms are considered as explicit couplings of second-quantized operators to open-shell singlets. Geminal wavefunctions are constructed, and their structure is elaborated. Numerical results for small model systems clearly demonstrate improvement over closed-shell singlet pairs.