Sixteen electronic states of the iridium trimer (Ir3)

The equilibrium geometries and energy separations of sixteen electronic states (of doublet, quartet, sextet, and octet spin multiplicities) of Ir3 are computed using the complete active space multiconfiguration self-consistent field (CASSCF) method followed by large-scale multireference singles + doubles configuration interaction (MRSDCI) calculations that included up to 4 million configuration spin functions (CSFs). The spin-orbit effects were included using the relativistic CI (RCI) method. Two nearly degenerate electronic states were found (8A2 and 8B1 with C2v symmetry) within 0.1 eV. The final results including spin-orbit effects are in accord with the observed ESR spectra, provided the spectra are due to Ir3.