Improved version of a local contracted configuration interaction of singles and doubles with partial inclusion of triples and quadruples

Abstract

A local contracted single and double configuration interaction (LC-CISD) method, which introduces contracted singly and doubly excited vectors within the framework of bond functions, has been recently proposed [P. Reinhardt et al., J. Chem. Phys. 129, 164106 (2008)]. The present work improves this method by introducing a coupled-electron pair approximation (CEPA-3) dressing and by incorporating the leading part of linked effects of triples (T) and quadruples (Q) through a series of local four-electron full CI calculations. Two different ways have been adopted to incorporate this linked TQ effect. One consists of dressing the first column/line of the whole LC-CISD matrix. The other one introduces an additional contracted wave function responsible for the linked effect for each bond pair. The present LC-CEPA-3+TQ treatments have been applied to the evaluation of equilibrium bond lengths and harmonic frequencies of diatomic molecules (HF, BF, CuH, N(2), F(2), and Cl(2)) and single bond breaking in HF, CH(4), ClCH(3), ClSiH(3), n-butane, and F(2) molecules, symmetrical stretching of the two OH bonds in a water molecule, and symmetrical expansion of a triangular Be(3) cluster. The results show that the performance of the LC-CEPA-3+TQs compares favorably with coupled-cluster singles and doubles (CCSD) and CCSD(T) methods, presenting similar behaviors around equilibrium and better ones for stretched geometries. The LC-CEPA-3 method is strictly separable, and the size consistency error of our treatment of triples and quadruples is extremely small. The strict separability can be further achieved by dressing the doubly excited bond functions with the linked TQ effect. The efficiency of truncations on the bielectronic integrals has also been tested.