To me, every statistical ensemble in statistical physics was introduced beginning with the microcanonical ensemble, in which every microstate is equally probable. A canonical ensemble is described by combining two ensembles, who together shall form a microcanonical ensemble. The microstates in system 1 shall then form the canonical ensemble, system 2 is said to be large compared to system 1, so that it's temperature $T = \frac{\partial S}{\partial E}$ doesn't change when the two systems interchange energy. If one still requires every possible microstate of the whole system to be equally probable, then the probabilities for microstates $\Gamma$ in the small system scale with a factor $e^{-\frac{H(\Gamma)}{k_b T}}$. The canonical ensemble is no longer described by its energy, but by its temperature ( and volume V, particle number N ....)
My question: Is there also a way to describe an ensemble that has a fixed Energy, E, but varying Volume, that means, a EpN Ensemble, or a SpN Ensemble? That would mean that I look at two systems that can interchange Volume, but don't interchange energy, in the same way I described it above for the canonical ensemble.
I am asking because the Enthalpie H(S,p,N) exists, and is a thermodynamical potential that depends on S,p,N, which suggests that such an ensemble exists.
