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 F2, the bond stretching of water and the inclusion of an Be atom in the H2 molecule. Comparisons with approximate methods such as average quadratic coupled cluster (AQCC) are presented. AQCC appears as a good approximation to (SC)2.