Casimir-Polder forces from density matrix formalism
  We use the density matrix formalism in order to calculate the energy level shifts, in second order on interaction, of an atom in the presence of a perfectly conducting wall in the dipole approximation. The thermal corrections are also examined when $\hbar \omega_0/k_B T = k_0 \lambda_T \gg 1$, where ${$\omega_0=k_0 c$}$ is the dominant transition frequency of the atom and $\lambda_T$ is the thermal length. When the distance $z$ between the atom and the wall is larger than $\lambda_T$ we find the well known result obtained from Lifshitz's formula, whose leading term is proportional to temperature and is independent of $c$, $\hbar$ and $k_0$. In the short distance limit, when $z\ll\lambda_T$, only very small corrections to the leading vacuum term occur. We also show, for all distance regimes, that the main thermal corrections are independent of $k_0$ (dispersion is not important) and dependent of $c$, which means that there is not a non-retarded regime for the thermal contributions. 
T.N.C. Mendes and C. Farina