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.
[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.
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