Many-electron systems, SAPT

The SRS perturbation theory is the simplest of all SAPT expansions proposed thus far, and in view of numerous recent applications to complexes of direct experimental interest (see Refs. (1, 31) for reviews) it is important to study the convergence properties of this expansion, and its applicability in low orders to interactions of many-electron systems. When... 

Sections 2-4 presented the SAPT approach assuming that the SchrOdinger equation for the monomers can be solved exactly or nearly exactly. This is not the case for many-electron systems and therefore an extension of SAPT to these systems requires further theoretical developments. In two-body, many-electron SAPT, one uses the following partitioning of the total Hamiltonian ... 

In principle, the theory reviewed in Sections 4-6 can be applied to interactions of arbitrary systems if the full configuration interaction (FCI) wave functions of the monomers are available, and if the matrix elements of H0 and V can be constructed in the space spanned by the products of the configuration state functions of the monomers. For the interactions of many-electron monomers the resulting perturbation equations are difficult to solve, however. A many-electron version of SAPT, which systematically treat the intramonomer correlation effects, offers a solution to this problem. 

A further difficulty arises because the exact waveflmctions of the isolated molecules are not known, except for one-electron systems. A common starting point is the Hartree-Fock wavefunctions of the individual molecules. It is then necessary to include the effects of intramolecular electron correlation by considering them as additional perturbations. Jeziorski and coworkers [66] have developed and computationally implemented a triple perturbation theory of the symmetry-adapted type. They have applied their method, dubbed SAPT, to many interactions with more success than might have been expected given the fundamental doubts [67] raised about the method. SAPT is currently both useful and practical. A recent application [68] to the CO2 dimer is illustrative of what can be achieved with SAPT, and a rich source of references to previous SAPT work. 

---