Low-lying electronic states of Ir(n) 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. Ir2 and Ir3 were also optimized using the CASSCF method. MRCI-SD (for Ir2) energies and CCSD(T) (for Ir2 and Ir3) 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 Ir2 is predicted to be (5)Δg, and the ground state of Ir3 is linear (2)Δg, with the D3h(4)A"1 state ~10 kcal/mol higher in energy at the CASSCF level without core-valence corrections and ~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 ~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.