POTENTIAL-ENERGY SURFACES FOR MO+CO AND W+CO

Potential energy surfaces for MoCO and WCO have been studied using the complete active space multiconfiguration self-consistent field (CAS-M CSCF) followed by multireference singles + doubles configuration inter action (MRSDCI). Spin-orbit effects are included through the relativis tic configuration interaction (RCI) method for W-CO. Although at the C ASSCF level the lowest (7) Sigma(+) state does not form a very stable minimum for WCO relative to W(S-7) + CO((1) Sigma(+)), at the higher M RSDCI level not only does this state form a minimum but the (5) Phi an d (5) Sigma(+) states become nearly-degenerate with (7) Sigma(+) and f orm stable minima for WCO. In contrast, the (7) Sigma(+) State of MoCO does not form a minimum at all levels of theory, although the quintet states have minima in the potential surfaces and are above Mo(S-7) CO((1) Sigma(+)). Relativistic effects are found to stabilize WCO in c ontrast to MoCO. The lowest spin-orbit states arising from W(D-5) and W(S-7) are reversed in energy when spin-orbit effects were included co nsistent with the experimental atomic energy separations. The 0(+) sta te of WCO was found to undergo an avoided crossing due to spin-orbit c oupling. The nature of bonding is discussed using the wave function co mposition and the Mulliken population analysis.