Talk:Configuration interaction

Comment

Isn't this article too narrowed/particularized to variational methods? I think the configuration interaction refers in general to take basis functions as linear combinations of slater determinants, but the article as it is written now seems to restrict to some particular variational treatment with a restricted basis set, "variational wavefunction", etc. Can somebody suggest a reference giving a more general treatmet of C-I? —Preceding unsigned comment added by 164.54.95.251 (talk) 20:22, 27 January 2011 (UTC)

I think the use of the superscript SD here should be changed as it normally does not indicate spin orbitals, but rather configurations that are single and double excitations from the Hartree-Fock reference configuration. What do others think? I am happy to make the changes and also add material about CIS and CISD. Bduke 04:55, 31 October 2005 (UTC)

Well, I created this page, but I don't have the time to expand it right now. So go ahead and improve it if you'd like. Be bold! Karol 09:46, 31 October 2005 (UTC)

Thanks, Karol. I have had a go at it. Feel free to correct it. Bduke 01:50, 1 November 2005 (UTC)

units

I'm trying to figure what most of this means from the bottom up, and so I have a question. What are the units (if any) for Ψ? 65.78.17.194 01:44, 9 October 2006 (UTC)

Ψ doesn't have any units: the value of Ψ² at a point gives the probability of the electron being located at that point, so it doesn't have units either. Ian 07:56, 30 July 2007 (UTC)

Ψ actually has units of inverse length/area/volume, depending on whether or not you have a 1/2/3 d wavefunction. Ψ² dV gives the probability of a particle being found within a volume dV. Probability is unitless, thus Ψ would have units of 1/L³ in this case. Itamblyn (talk) 21:31, 17 April 2009 (UTC)

history

who first developed this method?is it older than Moller Plsset perturbation theory?221.133.36.14 (talk) 23:27, 10 March 2009 (UTC) lili

Nuclear Physics

There are at least three major CI codes in nuclear physics (as opposed to atomic physics / quantum chemistry as currently discussed in the article). These include: MFDn, BIGSTICK (formerly REDSTICK), and nushellx. There is a related code called trdens. I'm of a mixed mind about whether this is worth mentioning in the article so I thought I'd post it here. —Preceding unsigned comment added by 97.125.237.151 (talk) 00:11, 4 November 2009 (UTC)

What are the two meanings?

The second sentence says "Two meanings are connected to the term configuration interaction in this context." It is not at all clear what are the supposed two meanings. I think there is only one meaning - the linear combination of Slater determinants. The rest explains why (to account for electron correlation) and how.

Unless someone can describe the "two meanings", I propose to just delete this sentence. Dirac66 (talk) 02:12, 13 June 2011 (UTC)

I am not clear either about the two meanings, but there are two approaches (at least) to doing CI. There is the linear combination of Slater determinants, which the article starts with and then there is the linear combination of configuration state functions (CSFs), which the article discusses in the second paragraph. They give equivalent results but they are not identical. The GAMESS(US) code, which is my favorite for doing CI uses both approaches. The user has a choice. Maybe this was the original intent of the phrase "Two meanings". --Bduke (Discussion) 02:37, 13 June 2011 (UTC)

OK, this is one possible interpretation, which could be presented more explicitly in the article. But when you say "equivalent results", do you mean that there are two formalisms which lead to the same energy and wavefunction expanded in two different sets of basis functions (Slater dets and CSFs)? If so I doubt the difference is significant enough to describe as "two meanings". Or are the energy and wf actually different, analogous to RHF and UHF methods, which would be a real difference?

As for the "original intent", the original edit (introduced in two places 30 June 2005, 11:28 UT and still in the article in one place) suggests that the two meanings are 1) the linear combination of Slater dets (later changed to CSFs) and 2) "the mixing (interaction) of different electronic configurations (states)". I would not call this two meanings but rather the mathematical expression and the physical sense of one and the same meaning.

So the question now is either of these a real enough difference to retain in the article with a clearer wording? What do you think? Dirac66 (talk) 18:45, 13 June 2011 (UTC)

A CSF can be a single determinant or a fixed combination of determinants such as (A + B). If you use determinants you will also include (A - B) which is an excited state. So CSFs only give the state you are after, usually the ground state, but determinants give you several states. The energy and wavefunction for the one you want will however be identical.

I agree that the "original intent" was not important. So I think you are right to delete what you suggested. We can discuss the difference between using determinants and CSFs elsewhere. --Bduke (Discussion) 23:11, 13 June 2011 (UTC)

OK, I have deleted it, thanks for your help. And I found CSF defined in Levine (Quantum Chemistry 4th edn p.291). I knew the concept but somehow I had always called it a "symmetry and spin adapted combination of determinants". CSF is shorter so I will say that in future. Dirac66 (talk) 23:55, 13 June 2011 (UTC)