The canonical ensemble
- Page ID
- 5257
In analogy to the classical canonical ensemble, the quantum canonical ensemble is defined by
\[ \underline {\rho } \] | ![]() | \(e^{-\beta H}\) | |
\[f(E_i) \] | ![]() | \(e^{-\beta E_i}\) |
Thus, the quantum canonical partition function is given by
\[Q(N,V,T) = {\rm Tr}(e^{-\beta H}) = \sum_i e^{-\beta E_i}\]
and the thermodynamics derived from it are the same as in the classical case:
\[A (N, V, T ) \] | ![]() | \(-{1 \over \beta}\ln Q(N,V,T)\) | |
\[E (N, V, T ) \] | ![]() | \(-{\partial \over \partial \beta}\ln Q(N,V,T)\) | |
\[ P (N, V, T) \] | ![]() | \({1 \over \beta}{\partial \over \partial V}\ln Q(N,V,T)\) |
etc. Note that the expectation value of an observable \(A\) is
\[\langle A \rangle = {1 \over Q}{\rm Tr}(Ae^{-\beta H})\]
Evaluating the trace in the basis of eigenvectors of \(H\) (and of \(\underline {\rho } \) ), we obtain
\[\langle A \rangle = {1 \over Q}\sum_i \langle E_i\vert Ae^{-\beta H} \vert E_i \rangle = {1 \over Q}\sum_i e^{-\beta E_i} \langle E_i\vert A\vert E_i\rangle\]
The quantum canonical ensemble will be particularly useful to us in many things to come.