Variational perturbation theory for density matrices
Michael Bachmann, Hagen M. Kleinert, Axel Pelster · Physical Review A · 1999
We develop a convergent variational perturbation theory for quantum-statistical density matrices which is applicable to polynomial as well as nonpolynomial interactions. We illustrate the power of the theory by calculating the temperature-dependent density of a particle in the double-well potential to second order, and of the electron in the hydrogen atom to first order.