Coupled-electron pair

Coupled—Pair Functional (ACPF) and Coupled Electron Pair Approximation (CEPA). The simplest form of CEPA, CEPA-0, is also known as Linear Coupled Cluster Doubles (LCCD). 

Figure 3.9b portrays homolytic bond formation by the recombination of radicals and is accompanied by charge transfer from A to B. The radicals must be singlet coupled. The interaction of triplet-coupled electron pairs is repulsive and does not lead to bond formation. The reverse process describes homolytic bond cleavage and results in singlet-coupled free radicals. 

Coupled Electron Pair Approximation Botschwina and Sebald, Ref. 98. 

Fourth-order and Higher-order Terms.—Most methods currently employed in the study of electron correlation in atoms and molecules, for example the coupled-electron pair theory, limited configuration mixing, etc., may be regarded as third-... 

The relationship between the coupled-electron pair approximation (c.e.p.a.) and the many-body perturbation theory has been discussed in detail by Ahl-richs.149 All of the methods denoted by c.e.p.a. (x) (x=0, 1, 2, 3) may be related to the summation of certain classes of diagrams in the many-body perturbation theory to infinite order. For example, c.e.p.a. (0), which is Cizek s linear approximation or Hurley s c.p.a. (0) ansatz150 is equivalent to the summation of all double-excitation linked diagrams in the perturbation series. This is also denoted d.e.m.b.p.t. (double excitation many-body perturbation theory) by some workers.151 168... 

Parallel to these endeavors, work started in Germany on new concepts to account for electron correlation. The independent electron pair approach (lEPA) was developed by Ahlrichs and Kutzelnigg, followed a few years later by the CEPA (coupled electron pair approach).The relation of these methods to contemporary Moller-Plesset second order (MP2) and coupled cluster treatments is discussed in Ref. 60. Work on circular dichroism by Ruch and on the chemical shift by Voitlander showed the variety of ab initio problems treated. The special priority program of the DFG from 1966-1970 demonstrated the intended impact. 

Besides the mentioned aperiodicity problem the treatment of correlation in the ground state of a polymer presents the most formidable problem. If one has a polymer with completely filled valence and conduction bands, one can Fourier transform the delocalized Bloch orbitals into localized Wannier functions and use these (instead of the MO-s of the polymer units) for a quantum chemical treatment of the short range correlation in a subunit taking only excitations in the subunit or between the reference unit and a few neighbouring units. With the aid of the Wannier functions then one can perform a Moeller-Plesset perturbation theory (PX), or for instance, a coupled electron pair approximation (CEPA) (1 ), or a coupled cluster expansion (19) calculation. The long range correlation then can be approximated with the help of the already mentioned electronic polaron model (11). 

Ahlrichs, R., Many body perturbation calculations and coupled electron pair models. Comp. Phys. Comm. 17, 31 5 (1979). 

CEPA Coupled Electron Pair Approximation for including dynamic correlations. 

The multi-reference coupled electron pair approximation (MRCEPA) has a long pedigree as one of the more rigorous quantum chemical procedures. A recent development, the state specific version (SS-MRCEPA) has been... 

Computation of Excited State Potential Energy Surfaces via Linear Response Theories Based on State Specific Multi-Reference Coupled Electron-Pair Approximation Like Methods (S. Chattopadhyay, D. Pahari, U. Mahapatra D. Mukherjee)... 

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