What is the density operator for an isothermal–isobaric ensemble (T,p,N)? In the microcanonical ensemble $(E,V,N)$, the density operator is
$$\hat{\rho}=\frac{\delta(\hat{H}-E\,\hat{I})}{Tr(\delta(\hat{H}-E\,\hat{I}))}$$
Where $\hat{H}$ is the Hamiltonian of the system and $\hat{I}$ is the identity operator.
In the canonical ensemble $(T,V,N)$, the density operator is
$$\hat{\rho}=\frac{exp\{\hat{H}/(k_B\,T)\}}{Tr(exp\{\hat{H}/(k_B\,T)\})}$$
Where $k_B$ is Boltzmann constant.
In the macrocanonical ensemble $(T,V,\mu)$, the density operator is
$$\hat{\rho}=\frac{exp\{(\hat{H}-\mu\,\hat{N})/(k_B\,T)\}}{Tr(exp\{(\hat{H}-\mu\,\hat{N})/(k_B\,T)\})}$$
Where $\hat{N}$ is the particle number operator.
Now, in the isothermal–isobaric ensemble $(T,p,N)$ the expression for the density operator should be similar to the expression used in the canonical ensemble but with the Boltzmann factor multiplied by $exp\{-p\,V/(k_B\,T)\}$.
My question is: How do you write the density operator for an isothermal–isobaric ensemble $(T,p,N)$ in terms of operators? Shall the volume $V$ of the system be written as an operator?
 A: Not sure if this answers your quesitons, or if this is more of a lenghty comment, but here goes.
So for the grand canonical the particle operator $\hat{N}$ is defined through $\hat{N}|i, N\rangle = N|i, N\rangle$, and so $\hat{\rho} = \frac{1}{Z} \sum_i \sum_N e^{-\beta(E_i-\mu N)}|i, N\rangle\langle i, N| = \frac{\exp(-\beta(\hat{\mathcal{H}}-\mu\hat{N}))}{\mathrm{Tr}\exp(-\beta(\hat{\mathcal{H}}-\mu\hat{N}))}$
Now in the isothermal-isobaric we should have $\hat{V}|i, V\rangle = V |i, V\rangle$, i.e. $\hat{\rho} = \frac{1}{Z} \sum_i \int\mathrm{d}V e^{-\beta(E_i+pV)}|i, V\rangle\langle i, V| = \frac{\exp(-\beta(\hat{\mathcal{H}}+p\hat{V}))}{\mathrm{Tr}\exp(-\beta(\hat{\mathcal{H}}+p\hat{V}))}$
I don't think that $\hat{V}$ has to have anything to do with $\hat{x}$, $\hat{y}$, $\hat{z}$, but am not certain of this.
I didn't find any particularly good references, but Statistical Mechanics: Theory and Molecular Simulation by Mark Tuckerman (p. 401) does list the density operator etc. for the isothermal-isobaric ensemble (in fact the author avoids using the operator notation for both $V$ and $N$, preferring to leave these out of the trace as separate sums/integrals). 
