  The $f\to d$ transition model by Duan and co-workers [Phys. Rev. B {\bf 66},
155108 (2002); J. Solid State Chem. {\bf 171}, 299 (2003)] has been very useful
in interpreting the $f\to d$ absorption, emission and nonradiative relaxation
of light lanthanide ions in crystals. However, based on the assumption that the
$f^{N-1}$ core spin-orbit interaction is weak compared to $f\to d$ exchange
interaction, this model, in the original form, is not applicable to
interpretation of the $f\to d$ transitions of heavy lanthanide ions or actinide
ions in crystals. In this work the model is extended to cover the cases of
heavy lanthanides and actinides, where the spin-orbit interaction of $f$
orbitals may be stronger than the $f-d$ exchange nteraction.
  Keywords: lanthanide; f-d transition; model; spectrum; actinide; VUV;
