4 SELF-CONSISTENT DRESSINGS TO ACHIEVE SIZE-CONSISTENCY OF SINGLES AND DOUBLES CONFIGURATION-INTERACTION

Benefitting from the flexibility of intermediate effective Hamiltonians, we define four different dressings of the singles and doubles CI matrix which take into account the dominant effect of triple and quadruple excitations. Self-consistent versions of the dressing insure the strict separability of the energy for a system splitting into separate fragments (and therefore size-extensivity) provided that localized MOs are used. The relative advantages of the different versions are discussed.