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. This newly developed methods designated UGA-EP2, Sigma(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 unitary group. UGA-EP2 shows perfect agreement to its conventional counterpart analytically and numerically when applying to closed-shell species. The higher-order approaches Sigma(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. (C) 2010 Elsevier B. V. All rights reserved.