Low-Lying Electronic States of Irn Clusters with n=2-8 Predicted at the DFT, CASSCF, and CCSD(T) Levels

Low-lying structures of the small iridum clusters Ir-n (n = 2-8) were optimized using DFT methods. Ir-2 and Ir-3 were also optimized using the CASSCF method. MRCI-SD (for Ir-2) energies and CCSD(T) (for Ir-2 and Ir-3) energies of the leading configurations from the CASSCF calculations were done to predict the low-lying states. The normalized atomization energies (< AE >) for Ir-n (n = 2-8) were calculated at the CCSD(T) level up to the complete basis set (CBS) limit in some cases,using the B3LYP optimized geometries. The ground state for Ir-2 is predicted to be (5)Delta(g), and the ground state of Ir-3 is linear (2)Delta(g), with the D-3h (4)A(n)1 state similar to 10 kcal/mol higher in energy at the CASSCF level without core-valence corrections and similar to 15 kcal/mol higher at the CCSD(T)/CBS level with spin-orbit and core-valence corrections. Inclusion of the spin orbit corrections in the normalized bond dissociation energies < AE > for Ir-n is critical and will decrease the < AE > by similar to 15 kcal/mol for n >= 4. The < AE > for Ir-n increases as n increases in general, and the < AE > is far from convergence to the bulk value at n = 8. The average coordination number (CN) and average bond length for the, low energy Ir-n clusters are far from being converged to the bulk values by n = 8.