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      https://www.ias.ac.in/article/fulltext/pram/018/03/0261-0270

    • Keywords

       

      Coupled-cluster theories; many body theory; size-consistency; atoms; molecules

    • 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 (TT+) 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 (TT+), both non-variational and variational approaches lead to identical equations upto any given order inT andT+ inTT2 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, inTT2 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.

    • Author Affiliations

       

      Sourav Pal1 M Durga Prasad1 Debashis Mukherjee1

      1. Department of Physical Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta - 700 032, India
    • Dates

       
  • Pramana – Journal of Physics | News

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