Coupled Cluster Valence Bond Method: Efficient Computer Implementation and Application to Multiple Bond Dissociations and Strong Correlations in the Acenes

We describe an efficient implementation of the coupled cluster valence bond (CCVB) model. CCVB captures a certain essential part of the description of molecules with strong correlations (SC), which allows it to achieve correct energy profiles when covalent bonds are broken, while maintaining proper spin symmetry and size extensivity. To illustrate treatment of SC in bond breaking, we examine the symmetric dissociation of the sulfur allotropes S-6 and S-8 into triplet S atoms. To show applicability to larger systems and to explore whether CCVB can capture aspects of SC that arise in extended pi systems, we report results for a series of acenes up to 12 fused benzene rings, with active spaces of up to 228 correlated electrons. The lowest-energy CCVB solutions found for two of the largest acenes show signatures consistent with multi-electron SC and partial delocalization.