A.Dement'ev, A. Zaitsevskii, Yu. Kiselev
First V.A.Fock Meeting on Quantum and Computational Chemistry, Novgorod, 1998,No.62
ABSTRACT. Ab initio calculations on several low lying excited electronic states of the OsO4 molecule and the isoelectronic IrO4+ are performed within the quasirelativistic effective core potential approach. A series of different contracted gaussian basis sets rangnig from double zeta to (9s7p6d2f)/ [6s5p4d2f] Os,Ir,(5s5p2d)/[3s3p2d] O are employed. The geometry optimisation of the ground states was carried out at the CASSCF level. The excitation energies are obtained by the multipartitioning perturbation theory second-order calculations and the difference- dedicated CI technique; in the latter case, the CI space comprised ca. 105 numerically selected 'semiactive' excitations of CAS-like functions. The lowest allowed excitation from the ground state is identified with the transition to 1T2 state which is dominated by the promotion of an electron from the 1t1 (essentially ligand-localised) MO to the 2e one arising from the d AO of the central atom. The corresponding transition energy in OsO4 is estimated to be 34575 cm-1; this value is in excellent agreement with the experimentally observed energy 35000 cm-1 The spin-orbit splitting for all the states mentioned above are found to be rather small.