Talk:Møller–Plesset perturbation theory

message: Just added a few lines to begin with. Hope more will come in time. - Karol Langner
This page is  very bad. All the equations peresented here are just a basic Rayleigh-Schroedinger perturbation theory expressions. There is NOTHING specific about Moller-Plesset. No definition of perturbation, no solution for MP2 etc ...
One can easily remove all MP2, MP3,   replace words  Moller-Plesset  with  Rayleigh-Schroedinger  in  title in text 
and  the article would be  perfectly fine. —Preceding unsigned comment added by 170.140.189.10 (talk • contribs) 
:Please sign all comments on talk page by adding ~~~~. I agree that this article needs improvement, but you are being a little unfair. It defines the MP2 as the difference between the Hartree-Fock operator and the exact Hamiltonian. It explains that the first order perturbation only converts the sum of orbital energies to the Hartree-Fock energy so correlation energy only starts to be introduced at second order. It is likely to be chemists who come to this page and in general they do not know about Rayleigh-Schroedinger so there is need to describe it. The equations for the MP2. MP3 and MP4 energies are rather complex and I think the editors of this page wanted to keep things as simple as possible. However I do think the MP2 energy should be added and a link given a source for the MP3 and MP4 energies. Why not try to improve it yourself? --Bduke 22:37, 28 June 2007 (UTC)

message: You have added some great material to the MP article. Well done. I do however have a slight disagreement. Your development is not wrong, but it is not what seems to me standard. Most texts (e.g. Introduction to Computational Chemistry, Frank Jensen) define the perturbation as simply H - F, and this gives MP0 as the sum of orbital energies and MP1 adds the correction to give the HF energy. You have defined the perturbation such that MP0 is is the HF energy and the first order correction zero. We also need to be clear about the difference between the energy to n'th order and the n'th order correction. Jensen for example gives:-
:MP0 = E(MP0) = sum of orbital energies
:MP1 = MP0 + E(MP1) = E(HF)
Which way should we go with this? --Bduke 23:37, 29 June 2007 (UTC)
thread_title: MP2 energy and related matters

message: The article states that "This result is the Møller-Plesset theorem: correlation does not contribute in first-order to the exact electronic energy. In the section on the alternative formulation, it states "Obviously, the Møller-Plesset theorem does not hold". In both formulations, correlation only enters at second order, so why does the theorem not hold for the alternative formulation? Paul, can you clarify this? I have to say that I have never heard of this theorem and I suspect that most chemists have not. I can find no mention of it in chemistry texts. I have found more evidence that the alternative formulation is very common in chemistry texts. Indeed I can find no chemistry texts that use the original formation. I am going to edit the article to make this chemistry usage clearer and give references, when I have finally sorted out the references. While MP originated in Physics, it has been hijacked by chemistry. There are thousands of chemistry articles that use MP2. Many chemists will want to start learning about MP2 from this article so I think we should make it more user-friendly to chemists. I'll have a go without displacing the original formulation. --Bduke 08:32, 6 July 2007 (UTC)
replies: text: Hi Brian, what you could do is give the "chemistry formulation" first and then as a note the "physics formulation". I heard of the Moller-Plesset theorem at Lowdin's summer school in 1968, and I think I have heard it called that way more often since then, but off-hand I couldn't give a source. Since the difference between the two partitionings is close to trivial you seem to miss the point: E(1) = 0 (physics partitioning) and E(1) ≠ 0 (chemistry partitioning). No doubt you are aware of the very large body of physics papers (Dyson, Brueckner, Wick, Goldstone, Hugenholtz, Abrikosov, van Hove, etc.) on many-body theory? As far as I am aware they all use the second-quantized hole-particle formalism, which is equivalent to what I wrote. (I could write the article in 2nd quantization, too, but then surely most chemists will be lost).--P.wormer 12:40, 6 July 2007 (UTC)
replies: text:  Hi Paul, I'll make some changes today, all being well, and see what you think. I'm inclined to leave the original formulation first as, well, it is the original, but then stress the other formulation is in wide use by chemists and give more references. I guess I heard about the Moller-Plesset theorem at Lowdin's Summer School (I forget when I went - early 70s I think) too, but I do not recall it. It is the definition that I am concerned about. Does it say that electron correlation enters only at second order (in which case both formulations fit it) or does it say E(1) = 0 (in which case the alternative chemists formulation does not)? The article appears to define it as the first, but draws the second conclusion. I hope we do not use second quantization here as chemists would indeed be confused. I know some of the papers you mention, but as a chemist, find them hard and not too useful. --Bduke 21:58, 6 July 2007 (UTC)
replies: text:  As far as I am concerned the article is OK now. --P.wormer 07:53, 7 July 2007 (UTC)
replies: text: Thanks for much improving the wording. I do not want to belabour the point but I'm still concerned about the definition of the MP theorem. The article states it as "correlation does not contribute in first-order to the exact electronic energy". By the definition of correlation energy, correlation does not contribute in either formulation until 2nd order, not withstanding whether the first order correction is zero or not. Are we using "correlation" in a different sense, or should this definition be rewritten to something like "perturbation does not contribute in first-order to the electronic energy"? I would be happier if we could take the definition exactly from some source. --Bduke 08:09, 7 July 2007 (UTC)
replies: text:  In the "physics" partitioning the first-order perturbation correction is zero. However, since the perturbation is known as "correlation potential", I phrased it the way I did. It is a (small) advantage of the "physics" partitioning that Lowdin's definition of correlation (energy beyond HF) coincides with the MP-PT definition (energy beyond EMP0 ≡ EHF). In the "chemistry"  partitioning one must define energy beyond EMP1 to get Lowdin's definition.  I will change "correlation" to "correlation potential" in the formulation of the MP theorem. (Note that the "correlation potential" is defined a few lines before this statement). If you still don't like it you may change it to perturbation (if you like that better), because from the context it is clear that the perturbation is the correlation potential.--P.wormer 10:20, 7 July 2007 (UTC)
replies: text: That is indeed clearer. I would prefer a good source in case someone wades and in and tries to change everything. I guess we do know a lot between us, but we have to avoid original research. I have exhausted my sources at home. I'll try to spend some time in the library this week. Cheers, Bduke 10:59, 7 July 2007 (UTC)
thread_title: Møller-Plesset theorem

message: I did what I could to fix the typography to the standard en dash to connect pairs of names.  But the term "post-Hartree–Fock" is a bit unusual; I wrote it this way because that's what most sources do, though one also finds "post–Hartree-Fock" and "post Hartree–Fock".  I'm not sure what rules apply here, so I haven't worked on fixing that article yet.  Opinions?  Dicklyon (talk) 04:07, 2 April 2009 (UTC)
replies: text: Hilarious even without the en-dash thing: I keep reading Post-Hotree–Fuck. Tony (talk) 11:55, 25 March 2011 (UTC)
thread_title:  En dash 

message: Its important that readers get the "big pictures" of how Perturbation theory is different from DFT (for example); in particular, I find most people cannot differentiate a calculation done using B3LYP and one using MP2 -- past some elementary statements, of course.  — Preceding unsigned comment added by 144.32.128.14 (talk) 16:47, 28 December 2011 (UTC)
thread_title:  Primary difference with other methods 

message: Please correct that.  — Preceding unsigned comment added by 2001:628:2120:60B:292C:C586:FBC5:D166 (talk) 06:30, 7 June 2017 (UTC)
replies: text: Definition of  \tilde{v}  was removed by  the anonymous 68.50.13.192 on 9 April 2015 at 16:22.   --P.wormer (talk) 11:20, 7 June 2017 (UTC)
thread_title:  <math> \tilde{v} </math> is not defined in the final energy expression