We have carried out systematic CASSCF and CASPT2 calculations for the ground states of lanthanide trihalides, LnX(3) (Ln = Ce-Yb, X = F, Cl, Br, and I), using model core potentials to investigate the effects of static and dynamic electron correlations on the structures of these molecules. Dynamic electron correlation is found to be indispensable for obtaining reliable bond distances in these molecules. Static electron correlation has little effect on bond distances, although the ground states of LnX(3) (Ln = Pr, Nd, Pm, Sm, Dy, Ho, Er, and Tm) have more than two major electronic configurations. (C) 2009 Elsevier B. V. All rights reserved.