AB-INITIO EFFECTIVE CORE POTENTIAL CALCULATIONS ON LANTHANIDE COMPLEXES - BASIS-SETS AND ELECTRON CORRELATION-EFFECTS IN THE STUDY OF [GD-(H2O)(9)](3+)

Calculations on [Gd-(H2O)(9)](3+) at the restricted Hartree-Fock (RHF) , density functional theory (DFT) and Moller - Plesset 2 (MP2) levels were performed using an effective core potential (ECP) including 4f el ectrons in the core for gadolinium. An optimized valence basis set wit h different contraction schemes for the metal, and the STO-3G, 3-21G, 6-31G, 6-31G** and D95** basis sets for water molecules were used. Ge ometry optimization provided three main conformations: a minimum and t wo saddle points, each corresponding to a tricapped trigonal prism arr angement of the coordinated water molecules. The minimum energy confor mation is of D-3 symmetry and does not correspond to any known experim ental structure. The geometries of the two saddle points are of C-3h a nd D-3h symmetry, respectively, and correspond to two different crysta llographic structures. At the DFT/D95* (RHF/D95**) level these are re spectively 4.95 (4.92) and 6.97 (6.26) kcal mol(-1) above the global m inimum. The results show that the major structural features of the coo rdination cage observed in the crystallographic structures are mainly due to intramolecular interactions between the coordinated water molec ules. Intermolecular interactions, such as crystal packing forces and hydrogen-bond networks, should be responsible for the energy stabiliza tion of the C-3h and D-3h structures in the solid state. At the RHF le vel, the STO-3G basis set is very sensitive to the choice of basis set on the metal center and, even in the best cases, provides unsatisfact ory results. On the contrary, the 3-21G basis set provides, independen tly of the metal basis set, quite reliable geometries and conformation al energies in qualitative agreement with those obtained using polariz ed basis sets. The results from the polarized basis sets themselves ar e consistent; further, inclusion of the electron correlation causes on ly a little shortening of the gadolinium-oxygen bond distances with re spect to RHF and the conformational energies are substantially unaffec ted. These results highlight that calculations with the adopted ECP fo r the metal can provide reliable results in the study of gadolinium co mplexes also at the RHF level; further, a satisfactory representation of the potential energy surface of these systems can be obtained by 6- 31G single point energy calculations on optimized 3-21G geometries. ( C) 1997 Elsevier Science B.V.