A formulation of multiple‐reference CI with terms linear in the interelectronic distances. II. An alternative ansatz

An alternative ansatz for r12‐MR‐CI is given. The final formulas are much more easily obtained than was possible previously. The numerical performance is demonstrated with a model calculation of the ground state of helium using three different Gaussian basis sets (5s, 10s, 16s). With the two largest basis sets, the energies obtained with the new formulas, the formulas of Part I of this series of articles, and the formulas of the original formulation agree within 2 μEH. Since the new ansatz is much easier to handle mathematically, we recommend its usage for future work.