This allows us to assess the reliability of widely used all-electron calculations while validating our approach. For 3T triangulenes, typically used all-electron methods include CIS with perturbative double excitations (i.e., CIS(D)), equation-of-motion coupled-cluster singles and doubles (EOM-CCSD), similarity transformed EOM domain-based local pair natural orbital CCSD (STEOM-DLPNO-CCSD), and second-order algebraic diagrammatic construction (ADC(2)). These approaches highlight the critical role of double excitations in accurately describing ST inversion. Additionally, it was recently discussed that triple excitations are necessary for a precise treatment of singlet-triplet inversion. However, the rapid increase in the number of multiple excited configurations makes third-order studies using all-electron methods impractical for large systems. A possible workaround involves using methods that include perturbative triple excitation corrections, such as ∆CCSD(T). However, these perturbative corrections derived from triply excited determinants do not alter the wavefunction, which remains at the CCSD level. Conversely, the PPP model, with its minimal representation of π-electrons, is exceptionally well-suited for exploring the significance of higher-order excited configurations in systems as large as 3T triangulenes.