Approaching the basis set limit for transition metal compounds with highly polar bonds: a benchmark coupled-cluster study of the ScF3 and FeF3 molecular structures and spectra.

The molecular equilibrium geometries, quadratic and cubic force constants, vibrational frequencies, and infrared intensities of scandium and iron trifluorides are determined ab initio with a sequence of atomic natural orbital basis sets using the CCSD(T) treatment of electron correlation. The largest basis set of spdf ghi quality contains 462 contracted Gaussian functions. Relativistic corrections are applied to compute the equilibrium geometries and vibrational frequencies. The cubic force constants are used to estimate vibrational corrections to the effective r(g) internuclear distances determined in the gas electron diffraction experiments. The computed molecular properties are extrapolated to the complete basis-set limit. The predicted values are compared to the available experimental data; uncertainties and inconsistencies in these data are then discussed.