Abstract
A unitary group reformulation of electron propagator theory is used to derive the second‐order and 2ph Tamm–Dancoff self‐energy approximations for open‐shell applications. A highest weight representation of the reference state is chosen in order to facilitate matrix element evaluation, but two special cases precluded by this choice are also discussed. Detailed numerical calculations are described for the lithium atom, oxygen molecule, and the amidogen radical.