Intruder state

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]