We present two new modifications of the second-order polarization propagator approximation (SOPPA), SOPPA(SCS-MP2) and SOPPA(SOS-MP2), which employ either spin-component-scaled or scaled opposite spin MP2 correlation coefficients instead of the regular MP2 coefficients. The performance of these two methods, the original SOPPA method as well as SOPPA(CCSD) and RPA(D) in the calculation of vertical electronic excitation energies and oscillator strengths is investigated for a large benchmark set of 28 medium-size molecules with 139 singlet and 71 triplet excited states. The results are compared with corresponding CC3 and CASPT2 results from the literature for both the TZVP and the larger and more diffuse aug-cc-pVTZ basis set. In addition the results with the aug-cc-pVTZ basis set are compared to the theoretical best estimates for this benchmark set. We find that the original SOPPA method gives overall the smallest mean deviations from the reference values and the most consistent results.
Molecular Physics, 2015, Vol 113, Issue 13-14, p. 2026-2045