Abstract
In this paper certain correspondences have been shown among various formulations of coupled-cluster theories for many electron closed-shell systems. Specifically it is shown that (i) the energy functional using unitary ansatz of the form exp (T−T +) is exactly sameorder by order inT with the size-consistent energy functional 〈ψ|H|ψ〉/〈ψ|ψ〉 recently obtained by us in coupled-cluster framework; (ii) in the framework of unitary ansatz of the form exp (T−T +), both non-variational and variational approaches lead to identical equations upto any given order inT andT + inT∼T 2 approximation; (iii) variational procedure using our size-consistent energy functional or using the functional obtained in the framework of unitary ansatz (as envisaged by Kutzelnigg) leads to energy in both cases, inT∼T 2 approximation, for a total of quadratic powers inT andT +, same as Cizek’s linearised coupled pair many electron theory energy; (iv) in case of practical calculation of the energy through the variational procedures using our size-consistent energy functional and the functional in unitary ansatz framework, there is a loss of upper bound.
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Pal, S., Prasad, M.D. & Mukherjee, D. On certain correspondences among various coupled-cluster theories for closed-shell systems. Pramana - J. Phys. 18, 261–270 (1982). https://doi.org/10.1007/BF02847816
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DOI: https://doi.org/10.1007/BF02847816