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Not a long time ago in our galaxy tags and were merged and in the very discussion of the merge few raised their concerns about distinguishing between and . Geoff Hutchison mentioned (I quote):

Perhaps that distinction can be maintained, since one is about the actual theory and one is about carrying things out on a computer.

However, in order to maintain the distinction we first have to make it which is not so easy to do, since theory always comes first followed by applications. Thus, as Martin - マーチン mentioned,

is (almost always) in the scope of

Quite often indeed when I answer a question tagged just with the tag I add the one. Still there are some types of questions which are almost exclusively about real-world computations and have very little to do with the theory, such as, for instance, the questions on which keyword can one use to battle the SCF convergence issues in some program and the like. As well as there are questions which are primarily about the theory and has almost nothing to do with how it is implemented on computers.

Anyway, if we would like to have both and tags out there, we have to decide one way or another what is the distinction between those two and what are the guidelines on their usage. So, let us have a thread to discuss this matter more closely.

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    $\begingroup$ I kind of really hoped it will never ever turn up again, but you point is fair and we should do that. I think we need to include theoretical chemistry in the process. I have no time right now, but I'll try to state my thoughts soon. Thank you for bringing it up though, even though I would have liked to avoid the discussion. $\endgroup$ – Martin - マーチン Mar 12 '16 at 15:16
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    $\begingroup$ My silence indicates that I'm too tired to chime in the discussion, but I think the ideal consensus that comes from this post is what makes me not want to want two [quantum-chem] and [comp-chem] tags on one post. [Theoretical-chem] is pretty weird though. I'm still trying to figure what it covers that other tags don't. $\endgroup$ – M.A.R. Mar 12 '16 at 17:37
  • $\begingroup$ Just adding this for further reference: The merger of the quantum tags was discussed here. And we concluded, that there is indeed still a lot to do concerning theo chem. $\endgroup$ – Martin - マーチン Mar 14 '16 at 10:41
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I've thought quite a bit about the distinction between the different sub-disciplines, but this post and the linked one have revealed to me that I don't usually make a distinction between theoretical and quantum chemistry since I'm biased by doing almost entirely electronic structure. I almost never use "quantum chemistry" but "theoretical chemistry", and don't really distinguish between them, which is wrong, because I think theoretical chemistry encompasses areas beyond just quantum chemistry. I've included links to the respective Wikipedia articles, though this might just add to the overall confusion.

Theoretical chemistry

Theoretical chemistry deals with the physical foundations of chemistry through mathematical models, both exact and approximate. This includes quantum chemistry and electronic structure theory, kinetics, molecular biophysics, theoretical spectroscopy, "atomistic" models for bulk materials, and many other sub-disciplines I'm sure to be missing. I'm lumping both the creation of the mathematical models and their implementation together under this banner, though traditionally the focus is more on the former than the latter. The latter may still be ill-defined...

Quantum chemistry

Quantum chemistry is the use of quantum mechanics within chemistry, technically both experimentally and theoretically. I tend to think of this as nothing more than a synonym for electronic structure theory, but this is also incorrect. Probably most of modern theoretical chemistry falls here, and even classical models can be derived as limiting cases for quantum ones, though I wouldn't place the most common instances of molecular dynamics here.

Computational chemistry

Computational chemistry is the application of computational programs to chemistry, where those programs are the implementation(s) of ideas from theoretical chemistry. It is solely the use of tools to solve chemical problems, not the development of those tools. These applications don't have to be quantum mechanical in nature, either.

You've already touched upon this:

Still there are some types of questions which are almost exclusively about real-world computations and have very little to do with the theory, such as, for instance, the questions on which keyword can one use to battle the SCF convergence issues in some program and the like.

Opinions on tags (with examples)

The quantum and computational chemistry tags

From my short experience on Chem.SE, I am in agreement with everything in your post. I don't think should necessarily be within the scope of , and vice-versa, but this is also just a restatement.

The distinction between and is the most clear one in my mind, or perhaps here is where my bias really shows.

Here are good examples of tagging. I'm not considering whether or not (or any other tag) is appropriate here.

Here are some not-so-good ones.

It may appear that I'm just trying to retag everything, but specific examples help form the correct duck test.

The theoretical chemistry tag

By the logic above, the tag has the danger to be misused more easily than the others, since it is the broadest in scope. It might be easier to identify posts that don't fall under this tag. Perhaps bringing up specific examples will help us better define .

Here are examples of questions I feel should not have the tag.

Other than the above mis-tagged questions, I've seen few discussions about that would fall outside the scope of either or . Here are some that I feel are good examples for not falling cleanly either either or , but are theoretical in nature.

Now for some that we could debate about, for a multitude of reasons.

Again, these are just my opinions. I think I know what theoretical chemistry is, but I don't know what the duck test for theoretical chemistry is in the context of Chem.SE.

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    $\begingroup$ Is this the duck test you're looking for? $\endgroup$ – M.A.R. Mar 13 '16 at 9:24
  • $\begingroup$ Yes, thanks, I'll fill it out for some of these. Though, can/should one expect the people posting the questions to do the same? $\endgroup$ – pentavalentcarbon Mar 13 '16 at 18:10
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    $\begingroup$ @pentavalentcarbon Only once they've happened across it. The SE model seems to entail a lot of blundering about as each user learns the ropes -- I've never found any sort of coherent introduction to the mid- and high-level workings of the sites. $\endgroup$ – hBy2Py Mar 14 '16 at 13:43
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My thanks to Wildcat for bringing this up—it had been bothering me for a while now.

Thoughts off-the-cuff, acknowledging the massive workload represented by the retaggings proposed (TRE #3, anyone? I'm on board for a team tackling this, BTW):

  • To my knowledge, comprises (A) and (B) other stuff.

    • (B) comprises primarily
    • So: Burninate and retag posts tagged therewith using something more specific: or other.
      • This will probably lead to the situation where we have a handful of tags that apply to a very small number of questions, but I'd rather have that problem than the current overlapped generality.
  • I think is too general to be useful. In my experience, in common usage it's nearly synonymous with , but as a descriptive term it could be stretched to cover just about every question on the site.

  • Questions relating to the use of computational software packages are squidgy.

    • At what point do such questions fall better in the scope of SciComp.SE? I don't have a strong opinion at this point, but perhaps:
      • When a question focuses on the hardware, the implementation details of the software, or the numerical techniques used to carry out the computations it should be referred to SciComp.SE
      • When a question addresses the chemical significance of the methodological details or the chemical-scientific content of the results themselves, it should be kept here (tagged or , say?)
    • Do we even want to consider "how do I run this computation?" sorts of questions on-topic for Chem.SE at all?
      • On one hand, there is lots of relevant expertise here, so we have the potential to be a valuable resource
      • On the other hand, how many such questions hold general value, versus being specific assistance useful only for the OP? Most packages have other, likely better places such questions can be asked (e.g., dedicated user forums)
      • If we do want to answer these, I would think a or or similar would be best for such questions, to distinguish "How do I do this?" types from "What's the theory of this method?" types. Geoff Hutchinson notes that we already have for this.
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    $\begingroup$ Remember we have the "software" tag too. I'd suggest the last class of "how do I run" should go there. $\endgroup$ – Geoff Hutchison Mar 17 '16 at 16:21
  • $\begingroup$ @GeoffHutchison Noted! Overlooked that one. Thanks! $\endgroup$ – hBy2Py Mar 17 '16 at 16:50

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