The lanthanide-chlorine bond length, R-e, and the frequency of the symmetric stretching mode, (ν) over bar (a1g), of the (LnCl(6))(3-) octahedral defect clusters embedded in Cs2NaYCl6 have been calculated for all 14 Ce3+ to Lu3+ impurities in their ground 4f(n) electronic state using wave-function-based ab initio methods of solid state quantum chemistry which include relativistic effects and electron correlation within the (LnCl(6))(3-) defect clusters and quantum mechanical interactions between the (LnCl(6))(3-) electronic group and the Cs2NaYCl6 embedding host ions. The bond distance values obtained provide useful data to improve the Judd-Morrison model of the 4f-->5d energy separation by explicitly including the local distortions the Ln(3+) ions produce in the Cs2NaYCl6 host. The values of the structural parameters R-e and (ν) over bar (a1g), and their variation across the series have also been studied using simpler models of the embedding host (in vacuo and Madelung embeddings), which has revealed that host effects, particularly those associated with quantum mechanical interactions, strengthen the Ln-Cl bond (decreasing the bond distances and increasing the frequencies of the symmetric bond stretch) and are smaller towards the right end of the series. Electron correlation within the (LnCl(6))(3-) clusters also reduces the bond distance values, but this reduction increases going right from Ce to Lu; its effects on the frequencies of the symmetric stretching mode are negligible. The comparison of the theoretical results with the few available experiments is very satisfactory. (C) 2003 American Institute of Physics.