AB-INITIO STUDY OF THE LANTHANIDE AND ACTINIDE CONTRACTION

Results for the spectroscopic constants of the monohydrides, monoxides , and monofluorides of the lanthanide elements lanthanum and lutetium as well as the actinide elements actinium and lawrencium from all-elec tron and valence-only ab initio electronic structure calculations are presented. The valence-only investigations were carried out using ener gy-consistent pseudopotentials for the heavy atoms, Electron correlati on was accounted for by means of the coupled-cluster method, Multirefe rence averaged coupled-pair functional calculations were carried out t o describe effects due to the mixing of the energetic close-lying ns(2 -m)(n-1)d(m) (0 less than or equal to m less than or equal to 2) confi gurations of the lanthanide (n = 6) and actinide (n = 7) cations, The counterpoise correction was applied to estimate the size of the basis set superposition errors, Nonrelativistic as well as quasirelativistic calculations were performed, The latter included also corrections for spin-orbit effects for the actinide compounds derived from limited co nfiguration interaction calculations. The reliability of the pseudopot ential approach is demonstrated by a comparison to the results of corr esponding nonrelativistic and scalar-relativistic Douglas-Kroll-Hess a ll-electron self-consistent-field calculations performed with large ba sis sets, The influence of shell-structure effects, i.e. the filling o f the If and 5f shell, and relativistic effects on the molecular prope rties is discussed, The values for the lanthanide and actinide contrac tions are found to depend strongly on the ligand and to vary from 6 to ii pm for lanthanides and from 11 to 17 pm for actinides, Relativisti c effects play a significant role; for example, their neglect even lea ds to a slight lanthanide/actinide expansion in the case of the monoxi des.