Size extensivity iterative corrections

Paldus, elsewhere in this book, discusses that there is as yet no generally applicable, open-shell, size-extensive, coupled cluster method, and the same holds for open-shell S APT methods. Therefore, for the computation of potentials of open-shell van der Waals molecules one has the choice between CASSCF followed by a Davidson-corrected MRCl calculation of the interaction energy, or the single reference, high spin, method RCCSD(T). When the ground state of the open-shell monomer is indeed a high spin state, then RCCSD(T) is the method of choice. With regard to the latter method we recall that a major difficulty in open-shell systems is the adaptation of the wave function to the total spin operator S for the CCSD method a partial spin adaptation was published by Knowles et al. [219,220] who refer to their method as partially spin restricted . When non-iterative triple corrections [221] are included, the spin restricted CCSD(T) method, RCCSD(T), is obtained. 

Although BWPT looks conceptually simple, it has the disadvantage that the effective Hamiltonian depends in this case on the exact energy, which means that correlation corrections can only be calculated iteratively to get a self-consistent solution. A direct consequence of this is that only SRS PT is possible for BWPT. Another disadvantage is that not all orders of BWPT are size-extensive. 

---