Structural and vibrational frequency studies have been carried out for 30 lanthanide trihalide molecules LnX(3) (Ln = La-Lu; X = Cl, F) using ab initio quasi-relativistic effective core potentials at Hartree-Fock and Moller-Plesset post-Hartree-Fock levels of theory. A planar D-3h geometry was obtained for all trichloride molecules. In accordance with previous theoretical works, a C-3v, structure was clearly established for the majority of the trifluorides. Calculated bond lengths and vibrational frequencies are in good agreement with the experimental data. Therefore, thermochemical functions have been estimated from thermostatistics calculations combined with our quantum mechanics results. The electronic partition function is crucial in order to be in accordance with experimental data tables, especially for temperatures higher than 500 K. A simple example of thermochemical equilibrium clearly proves the validity of the estimated functions in the case of the lanthanum halides.