Size-consistent self-consistent configuration interaction from a complete active space

The size-consistent self-consistent (SC)(2) method is based on intermediate Hamiltonians and ensures size-extensivity of any configuration interaction (CI) by correcting its diagonal elements. In this work, an (SC)(2) dressing is proposed on a complete active space SDCI. This approach yields a more efficient code which can treat larger multireference problems. Tests are proposed on the potential energy curve of F-2, the bond stretching of water and the inclusion of an Be atom in the H-2 molecule. Comparisons with approximate methods such as average quadratic coupled cluster (AQCC) are presented. AQCC appears as a good approximation to (SC)(2). (C) 1998 Elsevier Science B.V.