**Abstract**

Ab initio molecular orbital calculations are performed for a series of
lanthanide trihalides LnX_{3} (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 (LnX_{3}), the planar
structure (D_{3h} symmetry) is calculated to be stable through
normal mode analyses at the complete active space self-consistent
field (CASSCF) levels. In the LnX_{3}, 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 LnCl_{3}, 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 *L*–*S* coupling.