Theoretical study of hydration models of trivalent rare-earth ions using model core potentials

We performed geometry optimization calculations and ab initio molecular dynamics calculations for hydration models of trivalent ions, Y3+, La3+, Gd3+, and Lu3+, by using molecular orbital calculations to examine the so-called gadolinium break. The geometry optimization calculations revealed that the M-O distances of Y(H2O)83+, Lu(H2O)83+, and La(H2O)93+ calculated at the MP2 level were 2.39 Å, 2.36 Å, and 2.63 Å, respectively, which agreed well with observed values. The calculated Gd-O distance (2.44 Å) of Gd(H2O)83+ also agrees reasonably well with the existing observed values. The differences in the M-O distances arise from the differences in the ionic radii. When the coordination number increased, the degree of decrease in the M-O binding energies per bond (B.E.) occurred in the following order: La3+ < Gd3+ < Y3+ < Lu3+. This order is consistent with the order of the ionic radii's inverse. In ab initio molecular dynamics calculations, by using the Y(H2O)243+ model we could realize the hydration structure where eight water molecules are located around the Y3+ ion. By using the bigger model of La(H2O)643+, the experimentally suggested structure coordinated by nine water molecules was reproduced.