Single determinantal zero-order

States with Single Determinantal Zero-Order Wavefunctions... 

COMMENTS ON FACTS FROM THE THEORY AND COMPUTATION OF GROUND STATES WITH SINGLE DETERMINANTAL ZERO-ORDER WAVEFUNCTIONS... 

The hypothesis that the electronic structures of states of atoms and molecules should be understood and computed within a framework that describes them in zero order in terms of Fermi-seas rafher than in terms of fhe conventional single configurations was first tested [7, 26b, 26c, 45] in terms of ifs physical relevance on the long-held view in research papers and in books that all noble gases are, par excellence, single-determinantal closed-shell atoms. Such a zero-order description leads to the conclusion that their properties should be roughly the same (hence their common epithet) and that the formalism and efficient methods for the computation of their properties should also be the same. 

Figure 8 Second order correlation energy diagrams for a closed-shell system described in zero order by a single determinantal Hartree-Fock wave function constructed from canonical orbitals. The one Brandow diagram of this type is shown in (a). The exchange diagrams are shown in Goldstone form in (b)...

CASPT2 is most useful for calculations on excited states and diradicals, where multireference wavefunctions are required. However, there are methods available for including electron correlation for radicals and radical ions for which single-determinantal wavefunctions represent good zero-order approximations, without resorting to multideterminantal (i.e., CASSCF) reference wavefunctions. Two of these methods are discussed in the following sections, and we recommend them over CASPT2 for most calculations on molecules with just one unpaired electron. 

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