Although there are only slight differences in the dominant configurations and weights between the RASCI wavefunctions in the (2h, 2e) and (3h, 3e) calculations, the excitation energies show a significant difference. This suggests that the states in this system are sensitive to minor changes in configuration or small variations in CI coefficients, indicating the potential impact of configurations with small CI coefficients not shown in Figure . Furthermore, this issue is specific to RAS calculations and would not arise in a typical CAS calculation. In the RASPT2 method proposed in the original study and implemented in this work, configurations that do not meet the criteria for RASCI restrictions are entirely excluded from consideration for electron correlation and are therefore not accounted for even in the perturbation treatment. Thus, when constructing the RAS space, it is crucial to ensure that all relevant configurations are included.