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?
- free molecule
- free molecular