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]

## References

1. ↑ 1.0 1.1 Glaesemann, Kurt R.; Gordon, Mark S.; Nakano, Haruyuki (1999). "A study of FeCO+ with correlated wavefunctions". Physical Chemistry Chemical Physics 1 (6): 967–975. doi:10.1039/a808518h. Bibcode: 1999PCCP....1..967G. https://web.archive.org/web/20170808050918id_/http://ccl.scc.kyushu-u.ac.jp/~nakano/papers/pccp-1-967.pdf.
2. ↑ Glaesemann, Kurt R.; Govind, Niranjan; Krishnamoorthy, Sriram; Kowalski, Karol (2010). "EOMCC, MRPT, and TDDFT Studies of Charge Transfer Processes in Mixed-Valence Compounds: Application to the Spiro Molecule†". The Journal of Physical Chemistry A 114 (33): 8764–8771. doi:10.1021/jp101761d. PMID 20540550. Bibcode: 2010JPCA..114.8764G.
3. ↑ Choe, Yoong-Kee; Witek, Henryk A.; Finley, James P.; Hirao, Kimihiko (2001). "Identifying and removing intruder states in multireference Mo̸ller–Plesset perturbation theory". The Journal of Chemical Physics 114 (9): 3913–3918. doi:10.1063/1.1345510. Bibcode: 2001JChPh.114.3913C.
4. ↑ Camacho, Cristopher; Witek, Henryk A.; Yamamoto, Shigeyoshi (2009). "Intruder states in multireference perturbation theory: The ground state of manganese dimer". Journal of Computational Chemistry 30 (3): 468–478. doi:10.1002/jcc.21074. PMID 18680217.
5. ↑ Gordon, Mark S.; Schmidt, Michael W.; Chaban, Galina M.; Glaesemann, Kurt R.; Stevens, Walter J.; Gonzalez, Carlos (1999). "A natural orbital diagnostic for multiconfigurational character in correlated wave functions". The Journal of Chemical Physics 110 (9): 4199–4207. doi:10.1063/1.478301. Bibcode: 1999JChPh.110.4199G. https://dr.lib.iastate.edu/handle/20.500.12876/14794.
6. ↑ Glaesemann, Kurt R.; Schmidt, Michael W. (2010). "On the Ordering of Orbital Energies in High-Spin ROHF†". The Journal of Physical Chemistry A 114 (33): 8772–8777. doi:10.1021/jp101758y. PMID 20443582. Bibcode: 2010JPCA..114.8772G.

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