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-rerefence 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, K. R.; Gordon, M. S.; Nakano, H. (1999). "A study of FeCO+ with correlated wavefunctions". Physical Chemistry Chemical Physics 1 (6): 967–975. Bibcode:1999PCCP....1..967G. doi:10.1039/a808518h.
  2. ↑ Glaesemann, K. R.; Govind, N.; Krishnamoorthy, S.; Kowalski, K. (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.
  3. ↑ Choe, Y. K.; Witek, H. A.; Finley, J. P.; Hirao, K. (2001). "Identifying and removing intruder states in multireference Mo̸ller–Plesset perturbation theory". The Journal of Chemical Physics 114 (9): 3913. Bibcode:2001JChPh.114.3913C. doi:10.1063/1.1345510.
  4. ↑ Camacho, C.; Witek, H. A.; Yamamoto, S. (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, M. S.; Schmidt, M. W.; Chaban, G. M.; Glaesemann, K. R.; Stevens, W. J.; Gonzalez, C. (1999). "A natural orbital diagnostic for multiconfigurational character in correlated wave functions". The Journal of Chemical Physics 110 (9): 4199–4207. Bibcode:1999JChPh.110.4199G. doi:10.1063/1.478301.
  6. ↑ Glaesemann, K. R.; Schmidt, M. 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.