Abstract
Two approaches to describing the thermodynamics of a subsystem that interacts with a thermal bath are considered. Within the first approach, the mean system energy is identified with the expectation value of the system Hamiltonian, which is evaluated with respect to the overall equilibrium distribution. Within the second approach, the system partition function is considered as the fundamental quantity, which is postulated to be the ratio of the overall and the bath partition functions, and the standard thermodynamic relation is used to obtain the mean system energy. Employing both classical and quantum-mechanical treatments, the advantages and shortcomings of the two approaches are analyzed in detail for various different systems. It is shown that already within classical mechanics both approaches predict significantly different results for thermodynamic quantities provided the system-bath interaction is not bilinear or the system of interest consists of more than a single particle. Based on the results, it is concluded that the first approach is superior.
- Received 3 March 2009
DOI:https://doi.org/10.1103/PhysRevE.79.051121
©2009 American Physical Society