I am wondering, however, whether we should have a separate tag for free-energy. I feel that the tag energy is really too vague, and as a matter of fact is one of the most used, because it can apply basically to anything, from general relativity to quantum field theory to fluid dynamics.
Free energy, on the other hand, belongs to a quite specific domain —thermodynamics/statistical mechanics—, and it is a very important concept there (Helmholtz free energy being the natural thermodynamic potential of the widely used canonical ensemble, and Gibbs free energy being fundamental in the study of phase transitions).
I think free energy deserves a tag of its own: energy is just too vague sometimes.
Examples of questions were I think that the tag free-energy would apply:
- Gibbs free energy intuition
- Why the free energy is called 'free'?
- Confusion about Free Energy and the Hamiltonian
- What exactly is temperature in the Gibbs free energy?
- Why is the Gibbs Free Energy F−HM?
- Proving that the free energy is extensive
...and many others.
These questions are specifically about (Gibbs or Helmholtz) free energy, not about energy in general.
I know that we should not encourage an uncontrolled proliferation of useless, too specific tags. However, being someone who is mostly active in thermodynamics and statistical mechanics posts on the site, I strongly feel that free-energy would be useful.
Of course, if we finally decide to undo the synonym, I volunteer to do the retagging of old questions and check when the tag can be applied to the new ones.
Addendum (to address the comments)
I think that adding statistical-mechanics or thermodynamics to energy is not enough to clarify, since in thermodynamics/statistical mechanics there is a thermodynamic potential called "(internal) energy" which is different from the free energy.
It's true that the tag may be misused by people asking questions about perpetual motion machine and other crackpottery. The best we can do on this front is write a clear usage guidance and correct tag mis-usages (after all there are other examples of tags which are useful but frequently misused, like mathematical-physics). However, we must also note that such questions are not so common on Physics SE and also usually they get closed pretty quickly.
I understand that we want to keep the tagging system simple. However, we also want it to be specific enough, and I think that this is one of the cases where additional specification is needed. Also, there are many examples of very specific tags that haven't been synonymized, like debye-length,quantum-teleportation,galaxy-rotation-curve,superalgebra etc.