Reduced multireference coupled cluster method IV: open-shell systems

The reduced multireference coupled cluster method with singles and doubles (RMR CCSD) is extended to handle high spin open-shell systems within the framework of the unitary group based coupled cluster theory (UGA CCSD). The method is applied to several states of multiply bonded diatomics, namely to the X (2)Sigma(g)(+), A(2)Pi(u), and B(2)Sigma(u)(+) states of N-2(+), and to X(2)Sigma and A (2)Pi states of CN. The performance and potentiality of the RMR CCSD approach are first tested at the double-zeta level of accuracy, considering stretched geometries and using various model spaces, particularly small ones, in which case the comparison with the exact full configuration interaction results can be made. Subsequently, the method is used to compute spectroscopic constants for several states of N-2(+) and CN, namely the equilibrium internuclear distances R-e, vibrational constants omega(e) and omega(e)x(e), and rotational constants B-e and alpha(e). Relying on the correlation consistent basis sets, there is very good agreement with experiment. In particular, the errors in the vibrational frequencies, when using cc-pVTZ basis sets, are usually within 10-20 cm(1).