Intruder state
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In quantum and theoretical chemistry, an intruder state is a particular situation arising in perturbative evaluations, where the energy of the perturbers is comparable in magnitude to the energy associated to the zero order wavefunction. In this case, a divergent behavior occurs, due to the nearly zero denominator in the expression of the perturbative correction.
Multi-reference wavefunction methods are not immune.[1][2] There are ways to identity them.[3][4] The natural orbitals of the perturbation expansion are a useful diagnostic for detecting intruder state effects.[5] Sometimes what appears to be an intruder state is simply a change in basis.[1][6]