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Calculations in the ground electronic state are probably the simplest cases where defaults in terms of charge (typically zero) and spin (singlet) are chosen by the calculation software. However, when dealing with reaction intermediates we encounter charged species with differing spin multiplicity. This radical may even correspond close to the first or second electronic state of the parent molecule, or a fragment and may require specified entry of the electronic state (as in CASSCF, RASSCF, etc).

What would be some good (general) resources to learn about modelling reaction intermediates (such as radicals). What are some of the caveats in such calculations. I am interested in methods such as DFT, MCSCF, MPn etc. for learning purposes.

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    $\begingroup$ +1, but what specifically are you asking for: a good method for charged or high-spin molecules? Or a good method for excited states? $\endgroup$ Jul 29 '20 at 12:26
  • $\begingroup$ @Nike Dattani Good method for charged and high spin molecules. $\endgroup$
    – ankit7540
    Jul 29 '20 at 13:56
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    $\begingroup$ Thanks, that makes it more clear, since the question says something about "2nd electronic state", and methods for excited states are very different from methods for ground states. Also you may have to be more specific about whether you're interested in "highly-charged" or "high-spin". Again the methods you'd use are different. For highly negatively charged, if using DFT, I might recommend DC-DFT. For high-spin I might recommend multi-reference methods depending on the situation. $\endgroup$ Jul 29 '20 at 14:00
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Theoretical studies of open-shell excited species is a wide open field of active and cutting-edge research. I would suggest you to look for papers in theoretical photochemistry, reviews such as this one, books, or even research workshops! In general, the density functionals should be used after serious testing for open-shell excited species.

In general, I would recommend three main books for understanding and getting a general overview of density and wavefunction-based methods

  1. Essentials of Computational Chemistry: Theories and Models: General overview about various applications and areas in computational chemistry.
  2. Introduction to Computational Chemistry: More details about implementation and formulations of computational methods.
  3. Molecular Electronic-Structure Theory: State-of-the-art textbook for wavefunction methods.

Note: I am in no way, shape, or form affiliated with Wiley&Co.

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  • $\begingroup$ +1. Not a bad start to the discussion. I just edit your post: please watch the spacing between words and punctuation marks, and the use of "an" when the following word doesn't start with a vowel. $\endgroup$ Jul 31 '20 at 14:26
  • $\begingroup$ @NikeDattani Thanks! Though one thing, the name of (3) is actually "Molecular Electronic‐Structure Theory" onlinelibrary.wiley.com/doi/book/10.1002/9781119019572 . $\endgroup$
    – mykd
    Jul 31 '20 at 14:28
  • $\begingroup$ I was wondering about that! I edited it back. Those Scandinavians are not native English speakers :) $\endgroup$ Jul 31 '20 at 14:31

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