Spin-adaptation of open-shell electron propagator theory

We derived the spin-adapted electron propagator theory complete through second-order which can be applied either closed or open-shell systems, excluding undesirable spin-contamination. These newly developed methods designated that UGA-EP2, Σ(2+) and DR2 (diagonal renormalized second-order approach) are single-determinantal and based on Unitary Group Approach (UGA)-Hamiltonian, which is expressed in terms of spatial orbital indices and exploiting spin-symmetry of many electron systems as a unitary group. UGA-EP2 shows a perfect agreement to its conventional counterpart analytically and numerically when applying to closed-shell species. The higher-order approaches Σ(2+) and DR2 can predict the experimental IPs accurately for several radicals, especially deeper states of ClO2 which is considered to be affected strongly by spin-contamination.