... Energies of perturbed states are then obtained as eigenvalues of effective Hamiltonian while projections of perturbed states onto zero-order states are defined by the corresponding eigenvectors. ... The model space was formed by the two states of interest. ... At the same time, there are large changes in the state energies in the low model space dimension regions of both MCQDPT2 and MCQDPT2 energies, and there is no saturation but rather a constant steady decrease of energies, up to n = 12. ... Chem...