A coupled-cluster study of lanthanum trihalide structures

Coupled-cluster calculations were carried at the CCSD(T)-level in order to determine the structures and vibrational spectra of the lanthanum trihalides LaF3 and LaCl3. Two different approaches were employed to describe scalar relativistic effects; the quasi-relativistic Effective Core Potential approach and the all-electron Douglas-Kroll approach. We find LaF3 to be of pyramidal structure and LaCl3 to be planar. The obtained vibrational frequencies accord well with experimental measurements except for the v(2) mode. This disagreement is attributed to the intrinsic uncertainty of harmonic frequencies of this fluxional mode as extracted from spectra which were obtained at high temperature and for matrix isolated species.