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I've been asked by a reviewer to change all occurrences of "free-molecule", which I use consistently throughout my manuscript, to "free-molecular". Typical usage examples: "particles colliding in the free-molecule regime", "calculations based on the free-molecule theory".

My question is, which of the possible four combinations below is grammatically correct/incorrect and why, and which would you recommend to use (my personal preference is 1)? Are there any rules to guide the choice?

  1. free-molecule
  2. free molecule
  3. free molecular
  4. free-molecular
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Please edit the question and add a few distinct sentences showing how you are using the phrase. Do you ever have any word other than regime after free-molecule? –  jwpat7 Dec 22 '11 at 0:25
@jwpat7: Checking in NGrams it seems by far the most common context for this expression is in fact the Free-molecular regime. Even if the word following is "theory", or "flow", for example, they're all basically about the same thing, so I don't think that matters. All that counts is the expression is adjectival. –  FumbleFingers Dec 22 '11 at 0:52
These four things may have different (connotation/meaning/frequency of use) in a particular technical area than they do in the general language. So I would assume the reviewer knows what is going on. –  GEdgar Dec 22 '11 at 1:39
@GEdgar: Let's face it, free molecules of any kind, doesn't mean much to most users of "general language". And like I said, all the specialist stuff is about the same thing - The Free-molecular aka Knudsen Regime –  FumbleFingers Dec 22 '11 at 2:29

2 Answers 2

There's no strong case for any one form being better than the others. I can't graph the hyphenated forms separately in this NGram - but they are included, and looking through a few pages it seems they're about equally common when the term is used adjectivally (the hyphen is rarely/never present when used as a noun, but surprisingly the noun usage isn't particularly common).

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So the short answer is this is a matter of stylistic preference. Whether OP should comply with his reviewer's request therefore depends on how strongly he prefers his original, and how important it is not to upset the reviewer.

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Merriam-Webster Unabridged says

  • gram molecule is a noun
  • gram-molecular is an adjective

Constructing by analogy:

  • free molecule is a noun
  • free-molecular is an adjective

Bottom line: free molecule is a noun, not an adjective. So the correct form is free-molecular, as in free-molecular regime (a regime of free molecules) or free-molecular theory (a theory pertaining to free molecules).

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If there's a discrepancy between what MW says and what they write in the specialist literature, I think you have to go on usage, not prescriptive grammar. There are probably more instances of free-molecule flow with a hyphen than without. The writers clearly intend free-molecule as an adjectival modifier to flow, not a noun. And as for gram molecular, I think they're plain wrong - over recent decades the hyphenless version seems to have become the most common. –  FumbleFingers Dec 22 '11 at 3:10
@FumbleFingers There's a distinct difference between gram molecule and gram-molecular. And free molecular, which is indeed an adjective (hyphenated or not), is what I see favored over free molecule (hyphenated or not) more often than not. –  Gnawme Dec 22 '11 at 5:09
@FumbleFingers Also, you said yourself in your comment under the OP's question that free molecular regime was the most common occurrence in the literature. Are you arguing against yourself now? –  Gnawme Dec 22 '11 at 7:13
I'm primarily saying that in situations like this, usage rather than dictionaries determines what's "correct". And in MW's specific case of gram-molecular, the dictionary does not reflect the current dominant form (where the hyphen is usually omitted in more recent stuff). With molecule/molecular, both versions are about equally common, and hyphenation is about evenly split as well, so I don't think you can really give an answer saying one combination is "right" based on analogy with something in MW that seems to be actually "wrong". –  FumbleFingers Dec 22 '11 at 16:20
@FumbleFingers My basic point is, free molecule is a noun, and the OP is trying to modify regime, etc. with it when an adjective (free molecular) is the part of speech to use. Any editor I've worked with would flag the OP's usage simply for that. –  Gnawme Dec 22 '11 at 17:31

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