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-rerefence 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.
Famous quotes containing the word state:
“Hast ever ben in Omaha
Where rolls the dark Missouri down,
Where four strong horses scarce can draw
An empty wagon through the town?
Where sand is blown from every mound
To fill your eyes and ears and throat;
Where all the steamboats are aground,
And all the houses are afloat?...
If not, take heed to what I say,
You’ll find it just as I have found it;
And if it lies upon your way
For God’s sake, reader, go around it!”
—For the State of Nebraska, U.S. public relief program (1935-1943)