Intruder states, backdoor

Fig. 2.4 In multireference Rayleigh-Schrodinger perturbation theory, states from outside the reference space, V, which assume an energy below that of any state among the reference set for — 1 < A < 0 are termed backdoor intruder states. Unlike the intruder states corresponding to 0 < X < +1, which often have a physical origin, backdoor intruder states are frequently unphysical...

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