Multi-reference coupled electron-pair

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)... 

Sakai et al. used several advanced post-Hartree-Fock methods (complete active space SCF, multi-reference singly- and doubly-excited conguration interaction, and multi-reference coupled pair approximation) in studies of low-lying electronic states of TiCl and ZrCl [209]. They found that the ground state for TiCl was and the lowest excited states were and (in that order). For ZrCl it was the doublet that was the ground state, with the excited states and S . The computed spectroscopic constants were reasonably close to the experimental ones. 

A brief introduction to coupled cluster theory has been given in Chapter 3, Section 3.3.2. Approximations based on cc expansions include, CCD, CCSD ccsdt, etc. and their multi-reference variants mr-ccd, mr-ccsd, mr-ccsdt, etc. The ccsD approximation is also known as the coupled-pair approximation (CPA) [75] or the coupled pair many electron theory (cpmet) [76,77]. 

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