Finite-temperature many-body perturbation theory in the canonical ensemble

Benchmark data are presented for the zeroth- through third-order many-body perturbation corrections to the electronic Helmholtz energy, internal energy, and entropy in the canonical ensemble in a wide range of temperature. They are determined as numerical $\lambda$-derivatives of the respective quantities computed by thermal full configuration interaction with a perturbation-scaled Hamiltonian, $\widehat{H}={\widehat{H}}_0+\lambda \widehat{V}$. Sum-over-states analytical formulas for up to the third-order corrections to these properties are also derived as analytical $\lambda$-derivatives. These formulas, which are verified by exact numerical agreement with the benchmark data, are given in terms of the Hirschfelder–Certain degenerate perturbation energies and should be valid for both degenerate and nondegenerate reference states at any temperature down to zero. The results in the canonical ensemble are compared with the same in the grand canonical ensemble.