Convergence of the multipole expansion of the polarization interaction
  The multipole expansion of the polarization interaction between a charged particle and an electrically charged particle has long been known to be asymptotic in nature, i.e. the multiple expansion diverges at any finite distance from the atom. However, it is shown that the multipole expansion of the polarization potential of a confined hydrogen atom is absolutely convergent at a distance outside the atoms confinement radius. It is likely that the multipole expansion of the dispersion interaction of two confined atoms will also be absolutely convergent provided the internuclear separation of the two atoms is sufficiency large to exclude any overlap between the electron charge clouds of the two atoms. 
Yong-Hui Zhang, Li-Yan Tang, Xian-Zhou Zhang, and J. Mitroy
physics.atom-ph