A recently developed compact effective core potential (ECP) scheme for lanthanides [T. R. Cundari and W. J. Stevens, J. Chem. Phys. 98, 5555 (1993)] is applied to molecular species-the lanthanide trihalides (LnX(3), Ln=Ce to Lu; X=F, Cl, Br, I). This research is the first ECP study of lanthanide trihalides in which 4f electrons are explicitly included in the lanthanide valence space. The high-spin 4f(n) configuration of Ln(III) is described using multiconfiguration (MC) and unrestricted Hartree-Fock (UI-IF) wave functions. Both methods give excellent agreement with experiment for Ln-X bond lengths, although UHF is faster with no evidence of extensive spin contamination. There is no loss of accuracy from early to late members of the lanthanide series and from the lightest to heaviest halogen for the MC calculations. The root-mean-square difference between MC/ECP Ln-X bond lengths and those estimated from gas-phase experiments is only 0.05 Angstrom (approximate to 2%). The UHF calculations on LnF(3) yield only three nonplanar minima (CeF3, PrF3, and PmF3). For the three complexes with pyramidal minima, the planar transition states are very close in energy (Delta E(UHF)<100 cm(-1)), suggesting these complexes will probably be fluxional at ambient conditions.