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There is this question: What exactly is meant by 'multi-configurational' and 'multireference'?

But I still don't understand what the multiple "references" are. I understand that you can for example use a HF Ansatz and exited states for multi-configurational methods. And that multi reference methods use more than one configuration to build exited states. But what are these configurations? There is only one HF Slater determinant, how do the other references look like?

I would appreciate if you could answer the questions with some additional formulas instead of just describing it by words :)

M0M0
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    The references are just other determinants that differ in their occupied/unoccupied orbitals. Multi-reference CISD would then generate all singly and doubly excited determinants starting from these references. It also allows you to include specific determinants without having to create all other higher excited determinants. This might seem strange when you think about a stationary calculation for one geometry but an example where it is useful is excited state molecular dynamics with surface hopping. There you need to describe many states adequately at very different geometries. – Hans Wurst Jul 23 '21 at 21:48
  • This question is more suited for https://mattermodeling.stackexchange.com/ imo. – S R Maiti Jul 24 '21 at 06:49
  • @HansWurst: But if they differ in their occupied/unoccupied orbitals, these are simply excited states, right? So they should be already included in CISD. – M0M0 Jul 27 '21 at 20:52
  • An electronic excited state is in the general case a linear combination of determinants. The determinants themselves form a "basis" into which we expand the electronic state. For singlet groundstates it is most of the time sufficient to use only a single determinant, but for excited states many determinants can contribute. And the basis set of determinants that are available for the expansion are build up by reference to an initial determinant. Further the determinants are generated by "exciting" this initial determinant. By adding a second reference, you will generate a larger basis. – Hans Wurst Jul 28 '21 at 21:42
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    I’m voting to close this question because it has been asked by the same user on Matter Modeling – Tyberius Aug 02 '21 at 21:30
  • @SRMaiti If you continue to suggest other sites, please make sure to also mention that cross-posting is generally frowned upon in the SE network. Be more helpful and avoid getting these questions closed for no good reason. – Martin - マーチン Aug 06 '21 at 22:41
  • @SRMaiti I also completely, utterly disagree with you. And your comment makes it sound that the question is off-topic here, which it is absolutely not. – Martin - マーチン Aug 06 '21 at 22:54
  • @Martin-マーチン I continue to suggest matter modelling, because computational chemistry questions keep getting closed here. I usually mention that cross-posting is frowned upon when an user responds to my comment. Also, you are the mod here, and you voted to close the question, I didn't, so accusing me of getting it closed is a bit unusual. There is nothing in my comment which would suggest it is off-topic here. – S R Maiti Aug 07 '21 at 11:52
  • @srmaiti I have closed the question as a direct consequence of it being cross-posted. I would not have had to do that if it wasn't. I could have migrated it, if that were the wish of the OP, but I do not think this would have been necessary at all. Your message, as well intentioned as it might have been, was quoted by the OP on matter modelling as the reason. So yes, in my view, you are directly responsible for it being closed here. – Martin - マーチン Aug 07 '21 at 21:43
  • @Martin-マーチン The question remained unanswered here for 15 days after being asked. On matter modelling it received an answer within 3 hours. That in itself is justification enough that computational chemistry questions are better asked on that site, and this has been demonstrated repeatedly in the past on other questions. The fact of the matter is that there are more modelling people on that site than on this site, so I will continue to suggest that site to computational questions. You are free to disagree but you won't change my mind. – S R Maiti Aug 08 '21 at 03:13
  • @SRMaiti You are confusing two things here: 1. Since MM is public I have not once questioned its validity; I have on multiple occasions chosen to recommend it myself. So while this would obviously have been a valid course of action we come to the real issue. 2. I am criticising the way you communicated your recommendation. It is rather obvious that it was conceived as a request rather than a recommendation. So I am asking you once again: When you suggest asking on another site, include a remark that cross-posting is discouraged, and be more friendly. Anything else: [chat] – Martin - マーチン Aug 08 '21 at 12:30
  • @Martin-マーチン Ok, I will keep that in mind, thank you. – S R Maiti Aug 08 '21 at 14:31

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