Variational Perturbation Theory for Density Matrices
  We develop convergent variational perturbation theory for quantum statistical density matrices. The theory is applicable to polynomial as well as nonpolynomial interactions. Illustrating the power of the theory, we calculate the temperature-dependent density of a particle in a double-well and of the electron in a hydrogen atom. 
M. Bachmann, H. Kleinert, and A. Pelster