Several low-lying electronic states of the ruthenium trimer (Ru-3) were investigated by complete active space multiconfiguration self-consistent field and multireference singles and doubles configuration interaction (MRSDCI) methods that included up to 45 million configurations. Two nearly degenerate states, namely, B-11(1) and B-11(2) states were found as candidates for the ground state at lower level of theory. At the highest MRSDCI level, the B-11(1) state was found to be the ground state of Ru-3. The atomization energy of this state was further determined with extensive large-scale MRSDCI calculations. Our calculations facilitate assignment of the recently observed resonance Raman spectra of Ru-3 in Ar matrix by Lombardi and co-workers. (C) 2003 American Institute of Physics.