There is a statement on page 60 of this dissertation:

the "physicist's water molecule" ($\ce{O-H = 1.1 \mathrm{\mathring{A}}, \angle HOH = 104^\circ}$)

I understand it to be a model similar to the "spherical cow in a vacuum,", in that it's meant to be an easy-to-remember approximation, but

  • What is the etymology of this phrase or definition?

  • Why this specific choice of parameters? It could just as easily be $\pu{1.0 \mathrm{\mathring{A}}}$, according to Wikipedia, and the angle could vary as well.

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    $\begingroup$ Googling the phrase "physicist's water molecule" yields the dissertation cited as the only hit. So it seem like a phrase the author invented. $\endgroup$
    – MaxW
    Nov 18, 2017 at 1:02
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    $\begingroup$ That's a (odd) phrase, not a definition. Obviously you cannot define a water molecule, physicist's or other's, as it is given by nature. $\endgroup$
    – Karl
    Nov 18, 2017 at 9:01
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    $\begingroup$ @Karl I disagree; I think it is a definition in the same way that linear water or planar ammonia are definitions. It is supposed to invoke some well-defined picture in your mind, even if it isn't physically sensible most of the time. $\endgroup$ Nov 18, 2017 at 22:10
  • $\begingroup$ (What would you do with a linear water molecule? Not even didactically useful.) I just checked the values for actual water, and they are 0.958 Angstroms and 104.45°. A spherical cow is a useful appoximation, as it simplifys modeling the cow. Using very imprecise values to stick into a computer simulation is however simply stupid imo, and using a wrong value like 1.1A is terribly stupid. I don't know what the author of that dissertation was thinking, but i wager it wasn't much. Tried and failed to sound clever, with that phrase. $\endgroup$
    – Karl
    Nov 19, 2017 at 8:32
  • $\begingroup$ (Or is he simply using those values as starting point to successfully refine better values? In that case I take back anything I said about stupid. Sorry, too lazy now to read the diss myself.) $\endgroup$
    – Karl
    Nov 19, 2017 at 8:34

1 Answer 1


I suspect the author might refer to "physicist's water molecule" term in order to underline the fact the geometry and energy of this molecule has been introduced as is, and hasn't been affected by intra- or intermolecular interactions.

I believe there is an allusion to Ancient Greece's philosophical schools which treated natural phenomena such as air, fire, water etc. as the purest and simplest entities. The term "physicist" itself refers to pre-socratic philosophers (Ionians, mostly) which abandoned religious attempts to formalize the reality [1, p. 20]:

... Pre-Socratic philosophers [...] have to be called Physicists because they abandoned the civil religion, whose insistency deified the city’s eponym, the royal function or the legislator’s role, the valiant warrior or the fertile woman, because they disdained what the social projects into the religious, whereas they kept or invented a global religious, that divine that’s immanent to the universe.

For instance, based on what's been considered the most fundamental element, physicist’s water has originally been introduced by Thales of Miletus, just as physicist’s fire "belongs" to Heraclitus.


  1. Serres, M.; Burks, R. Geometry: the third book of foundations; Bloomsbury Academic: London; New York, 2017,. ISBN 978-1-4742-8139-3. (Google Book)

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