Formulation of the Coupled Cluster Method for Quasi ID Polymers

In the case of periodic polymers in the T2T2 approximation of the coupled cluster method we can write7 

Here (pj(k, r) is the kth CO of the band j. Finally in equation (26) is a doubly excited Slater determinant in which was excited from the filled Wannier functions w,/f, wjjt, to the originally unfilled ones w, Wbj, . 

Substituting the expression (25) into the Schrodinger equation ) = ] F) one obtains 

This provides the correlation energy projected on the HF ground state as Hf — E + ( 72 0) = 0 

Projection of equation (29) on the space of doubly excited Slater determinants (expressed by Wannier functions) gives 

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