Multi-reference space

CC theory is inherently better than an equivalent level of Cl because it eliminates unlinked diagrams and as a consequence, is size-extensive [13]. It is also inherently better than an equivalent level of MBPT because it is not hmited to finite-orders, or potential difficulties encountered in the convergence of perturbation theory. It is well known, e.g. that ordinary MP perturbation theory does not converge for the electron gas, and this has also been emphasized recently for molecules [47], though resummations (including CC theory) work fine [48]. But, the operable word is equivalent level . For Cl, that meant at least single and double excitations, and frequently some more, perhaps even from a multi-reference space. MBPT had been done with single excitations in fourth-order SDQ-MBPT(4) in the above two papers [13,46]. 

Having decided about the basis set, we now turn to the problem of choosing an appropriate form of the wave function. Since we are going to calculate an energy difference between two electronic states, it is clear that we have to include as much correlation effects as possible. We shall do that in two steps In the first step we try to account for the near degeneracy effects by using the CASSCF method with an appropriate choice of the active space. On top of that we shall have to perform multi-reference Cl calculations to account for the dynamical correlation energy. 

---