Comparison of the quadratic configuration interaction and coupled cluster approaches to electron correlation including the effect of triple excitations

The recently proposed quadratic configuration interaction (QCI) method is compared with the more rigorous coupled-cluster (CC) approach for a variety of chemical systems. Some of these systems are well represented by a single-determinant reference function and others are not. We consider the infinite-order singles and doubles correlation energy, the perturbational triples correlation energy, and a recently devised diagnostic for estimating the importance of multireference effects. We have also calculated the spectroscopic constants of CuH, the equilibrium structure of cis-(NO) 2 , and the binding energies of Be 3 , Be 4 , Mg 3 , and Mg 4 using both approaches