Theoretical investigation of the alkaline-earth dihydrides from relativistic all-electron, pseudopotential, and density-functional study.

Highly precise ground state geometries, harmonic vibrational frequencies and force constants of alkaline-earth dihydrides from CaH₂ to RaH₂ are obtained using relativistic small-core energy-consistent effective core potentials at the coupled-cluster level. The results are compared with all-electron as well as density functional calculations. All-electron results, in particular, clearly show the importance of relativistic effects in the properties considered in this paper. The monotonic trends in the geometries are explained in terms of second-order perturbation theory. Trends in the force constants are monotonic except for the bending mode where an anomaly occurs from BaH₂ to RaH₂. It is rationalized in terms of reduced s-d hybridization due to relativity, which is shown to be an energy effect attributed to the stabilization of the s orbital. The pseudopotentials show an excellent performance in comparison with all-electron methods and are therefore successfully transferred to molecular cases. The density functional methods, however, suffer from functional dependencies with B3LYP performing the best in this case. [ABSTRACT FROM AUTHOR]