Talk:N-electron valence state perturbation theory

TODO (memo)
1) add a brief demonstration about the absence of intruder states, 2) provide wikipages to multipartitioning technique and Lindgren approach, 3) clarify the space definition for the strongly contracted approach and the involved operator 4) add a plot for the excitation classes 5) define the spin traced operators used in the PC theory --munehiro 21:36, 30 March 2006 (UTC)

Extending this page
I have added some comments on this page on User talk:Munehiro as he/she is the sole editor at present. Please consult them. --Bduke 22:02, 30 March 2006 (UTC)

Intruder States
You mention that standard (single state, single reference) MP2 avoids intruder states given its choice of denominators. However, I don't believe this is the case: in a nearly degenerate situation, some or all of the terms in the denominator can become nearly equal and so an 'intruder state' will appear. Chem-MTFC (talk) 13:59, 8 May 2008 (UTC)
 * Do you have a reference that supports your view? We can not add something just because someone does not "believe this is the case". --Bduke (talk) 23:13, 8 May 2008 (UTC)
 * If the orbital energies are similar (for a given state being treated perturbatively), then MP2 will give erroneous energies, since the denominator will become very small (r,s,i and j will be almost equal to each other). Thus MP2 can suffer intruder states.  There are various quasi-degenerate PT's that try to deal with this problem (for a single-reference HF wavefunction); see for example Theo. Chem. Acc. 105, 408 (2001); J. Phys. B 25, 603 (1992).  --Chem-MTFC (talk) 09:24, 12 May 2008 (UTC)

This article really does need to be expanded to mention the connection of this method to all other methods that carry out perturbation theory on a multireference wave function. --Bduke (talk) 23:13, 8 May 2008 (UTC)

Active Orbtials?
I'm not very familiar with this particular topic, and it would be good to make the use of key terms very clear. In this case, I don't think there is an obvious definition of "active orbitals" on this page.... presummably this is refering to the orbitals where the valence electrons are present? — Preceding unsigned comment added by 2.123.253.142 (talk) 17:46, 14 January 2012 (UTC)