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I want to do a PES scan of a radical structure in Gaussian. However, I'm not completely sure how to prepare an input file. I've figured out that charge and multiplicity are 0 and 2. What I don't know is how to indicate where a radical is. Will Gaussian figure it out itself by bonds and electrons count or do I have to indicate it explicitly?

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    $\begingroup$ Do you remember the postulate "The state of a quantum mechanical system is completely specified by its wavefunction"? Gaussian will find the wavefunction of your input system, which includes the location of the radical in that case (when dealing with wavefunctions, the spin density is the property you want to look at). Of course, in some cases you can (and should) do something "manually". For example, the 1-propyl radical will lack a hydrogen atom on the first carbon, while the 2-propyl radical will lack a hydrogen atom on the second carbon $\endgroup$ – The_Vinz Nov 20 '19 at 11:01
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    $\begingroup$ You are approximately solving the electronic Schrödinger equation. Using Gaussian means applying molecular orbital approximation. Radicals are not localised in this formalism. You cannot specify "where it is". $\endgroup$ – Martin - マーチン Nov 20 '19 at 11:02
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    $\begingroup$ In case it's not clear yet, Gaussian will figure it out itself. You just need to input the charge and multiplicity with your coordinates and other standard fare. $\endgroup$ – Blaise Nov 20 '19 at 12:52
  • $\begingroup$ Question for downvoters: what exactly is your problem? $\endgroup$ – Anna Nov 21 '19 at 10:30
  • $\begingroup$ @Anna I am not one of the down-voters, but I'd like to give some insight into why you might have attracted them. Broad computational questions which focus about a certain software are not popular with everyone. Conceptual questions (independent of the software) have a better stance. Your question also has a little also appears that you have not spent enough time with the program manual. Such questions are often seen as homework questions which lack in presenting enough own thoughts and effort. $\endgroup$ – Martin - マーチン Nov 21 '19 at 16:04