quantum-chemistry vs. computational-chemistry

Question

Not a long time ago in our galaxy tags quantum-mechanics and quantum-chemistry were merged and in the very discussion of the merge few raised their concerns about distinguishing between quantum-chemistry and computational-chemistry. 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,

quantum-chemistry is (almost always) in the scope of computational-chemistry

Quite often indeed when I answer a question tagged just with the computational-chemistry tag I add the quantum-chemistry 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 quantum-chemistry and computational-chemistry 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.

Answer 1

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 quantum-chemistry should necessarily be within the scope of computational-chemistry, and vice-versa, but this is also just a restatement.

The distinction between quantum-chemistry and computational-chemistry 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 theoretical-chemistry (or any other tag) is appropriate here.

• Complexity in computing the integrals involving higher angular momentum (quantum, theoretical)
• Brillouin's Theorem: How do other determinants mix with the ground state? (quantum)
• https://chemistry.stackexchange.com/questions/42359/how-does-casscf-yield-a-more-negative-total-energy-from-less-negative-orbital-en (quantum, computational)
• How to interpret orbital transition in TDDFT? (quantum, computational)
• Molecular mechanics force fields for metals? (computational)
• When using Gaussian to calculate NMR, what's the default solvent and frequency? (computational)

Here are some not-so-good ones.

• https://chemistry.stackexchange.com/questions/47585/how-to-fix-z-vector-failed-to-converge-in-gamess (should be computational, not quantum)
• Nuclear zero-point energy and quantum delocalization effects of water molecules (should not be computational)
• Programs for chemical structure visualisation and drawing 2d and 3d (should not be quantum and not even computational)
• Do bare protons exist, even transiently, in aqueous solution? (should not be computational)
• https://chemistry.stackexchange.com/questions/46386/wavefunction-convergence-error-and-keywords-spartan (should not be quantum)

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 theoretical-chemistry 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 theoretical-chemistry.

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

• Calculate the number of moles produced per hour per liter of air?
• https://chemistry.stackexchange.com/questions/47564/what-quantity-of-heat-is-required-to-heat-168-grams-of-copper-from-12-2oc-to-25
• Why is there sulfur in black powder / gun powder?
• How to calculate the concentration H3O+ in a solution with pH=6.99?
• What kind of math should I expect in first year chemistry?
• CASTEP Geometry Optimization Movie

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

• Calculating Raman Intensities from Static Raman Activities
• Radiation in space and its effects on chemical reactions?
• What is the significance of Law of Multiple Proportions?

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

• Acidity of unsaturated five membered ring relative to seven membered analogue
• Coordinates for molecules except cartesian and zmatrix
• Freezing all the water on earth with a seed crystal
• Software for predicting chemical reactions
• Charge consistency in fragment qm/mm methods
• How important is it that geometry be optimized at a high level of theory?

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.

Answer 2

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, computational-chemistry comprises (A) quantum-chemistry and (B) other stuff.

  • (B) comprises primarily molecular-mechanics
  • So: Burninate computational-chemistry and retag posts tagged therewith using something more specific: molecular-mechanics 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 theoretical-chemistry is too general to be useful. In my experience, in common usage it's nearly synonymous with quantum-chemistry, but as a descriptive term it could be stretched to cover just about every question on the site.

  • A synonym, chemical-theory, would be somewhat (but only slightly) less all-encompassing. It's still just too general: Aside from the pure observations of experiment, everything about chemistry involves the theories/hypotheses developed to explain those observations.

  • So: Burninate! Retag as necessary. For the examples given by pentavalentcarbon in his answer, I would (re)tag with things like

    • kinetics
    • spectroscopy
    • physical-chemistry
    • interstellar-chemistry/astrophysical-chemistry/astrochemistry
    • chemistry-mathematics

• 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 orbitals or electronic-structure, 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 computational-software or computation-execution 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 software for this.