A FORMULATION OF MULTIPLE-REFERENCE CI WITH TERMS LINEAR IN THE INTERELECTRONIC DISTANCES

A formulation of the extension of the method of Kutzelnigg and Klopper, to include terms that are linear in the interelectronic distances in the Ansatz for the wavefunction, to general multiple-reference CI (r12-MR-CI) with the same basis set requirements as in the closed-shell single reference case, is presented. The implementation into existing direct CI programs is straightforward. The additional CPU time needed by the r12 part should be negligible. r12-MR-CI is readily extended to r12-MR-ACPF, which allows for accurate approximations of full-CI energies. This implies the possibility of accurately solving the electronic Schrodinger equation for medium-sized atoms and molecules.