Bond breaking in quantum chemistry : A comparison of single-and multi-reference methods

Standard and new quantum chemical methods are evaluated for their ability to provide accurate potential energy curves for chemical reactions which break or form bonds. Comparisons to full configuration interaction benchmark results demonstrate that even high level single-reference methods such as unrestricted coupled-cluster with single, double, and perturbative triple substitutions [UCCSD(T)] can have large errors for bond breaking processes: nonparallelity errors for breaking bonds to hydrogen, which should be one of the simplest theoretical problems, are around 3-4 kcal mol -1 . Multi-reference methods are much more reliable but are also more computationally expensive. New, minimalist configuration interaction methods for bond breaking in larger molecules are also discussed which dramatically improve on similar earlier models.