The X-1 Sigma(+)(g), B-1 Delta(g), and B ` (1)Sigma(+)(g) states of C-2: A comparison of renormalized coupled-cluster and multireference methods with full configuration interaction benchmarks

Abstract

Unusual bonding and electronic near degeneracies make the lowest-lying singlet states of the C2 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 , B , and B′ states of C2, which are numerically exact solutions of the electronic Schrödinger 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 Å, it compares favorably with the more complicated multireference methods