Obviously, correlated ab-initio approaches such as coupled cluster theory (CC2, CCSD, and CC3), the algebraic diagrammatic construction through second order (ADC2), and complete active space second order perturbation theory (CASPT2) deliver reliable absorption energies in accordance with experimental results. However, these approaches are computationally quite expensive for tackling a conjugated molecular system beyond the basic PP. Other prominent approaches, such as symmetry adapted cluster-configuration interaction (SAC-CI) and similarity transformed equation-of-motion coupled-cluster (STEOM-CC) are more accurate than CASPT2, but limited to molecular system up to 50 atoms due to their high computational cost. To overcome the limits of CASPT2, second order N-electron valence state perturbation theory (NEVPT2) has been introduced, which is more efficient than CASPT2, size consistent and intruder-state-free, but like all multi-reference approaches, the computational cost of NEVPT2 is still high for larger systems. Overall, previously mentioned approaches yield reliable absorption energies for PPs, but suffer from a high computational cost for increased molecular systems, making them practically unsuitable for large molecules and materials.