The X1Σ+g, B1Δg, and B′ 1Σ+g states of C2:: A comparison of renormalized coupled-cluster and multireference methods with full configuration interaction benchmarks -: art. no. 124104

Unusual bonding and electronic near degeneracies make the lowest-lying singlet states of the C-2 molecule particularly challenging for electronic structure theory. Here we compare two alternative approaches to modeling bond-breaking reactions and excited states: sophisticated multireference configuration interaction and multireference perturbation theory methods, and a more "black box," single-reference approach, the completely renormalized coupled-cluster method. These approximate methods are assessed in light of their ability to reproduce the full configuration interaction potential energy curves for the X (1)Sigma(+)(g), B (1)Delta(g), and B-' (1)Sigma(+)(g) states of C-2, which are numerically exact solutions of the electronic Schrodinger equation within the space spanned by a 6-31G(*) basis set. Both the multireference methods and the completely renormalized coupled-cluster approach provide dramatic improvements over the standard single-reference methods. The multireference methods are nearly as reliable for this challenging test case as for simpler reactions which break only single bonds. The completely renormalized coupled-cluster approach has difficulty for large internuclear separations R in this case, but over the wide range of R=1.0-2.0 A, it compares favorably with the more complicated multireference methods.