Ab initio molecular orbital calculations are performed for a series of lanthanide trihalides LnX3 (Ln = La to Lu; X = Cl, F), with the relativistic effective core potentials of Cundari and Stevens, to characterize the tendency in their electronic and geometric structures. In all the complexes (LnX3), the planar structure (D3h symmetry) is calculated to be stable through normal mode analyses at the complete active space self-consistent field (CASSCF) levels. In the LnX3, the number of 4f-electrons increases with increasing the atomic number, and 1.2–1.6 (2.1–2.2) electrons are transferred from Ln to Cl (F); the Ln–X bonds are dominated by charge-transfer but have a significant amount of covalent character that involves the 5d-orbital on Ln. It is also found that, along the lanthanide trihalide series, the first seven f-electrons occupy 4f-orbitals one by one from the lowest one up, while the second seven occupy 4f-orbitals from the highest one down, at the Hartree–Fock level. This occupation mechanism is explained in terms of the self-repulsion interactions between two electrons occupying the same spatial 4f-orbital. The Ln–X bond lengths, net charges, and vibrational frequencies show monotonic variation along the lanthanide series, which corresponds to the lanthanide contraction. State-averaged CASSCF calculations are also carried out for LnCl3, in a combination with spin-orbit calculations using the atomic spin-orbit coupling constant for the f-electrons, to investigate the energy splitting of the nearly-degenerate low-lying states in the scheme of LS coupling.