In
quantum and
theoretical chemistry
Theoretical chemistry is the branch of chemistry which develops theoretical generalizations that are part of the theoretical arsenal of modern chemistry: for example, the concepts of chemical bonding, chemical reaction, valence, the surface ...
, 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.
There are ways to identity them. The natural orbitals of the perturbation expansion are a useful diagnostic for detecting intruder state effects. Sometimes what appears to be an intruder state is simply a change in basis.
References
Perturbation theory
Theoretical chemistry
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