Coupled-cluster doubles model

C. D. Sherrill, A. I. Krylov, E. F. C. Byrd, and M. Head-Gordon, /. Chem. Phys., 109, 4171 (1998). Energies and Analytic Gradients for a Coupled-Cluster Doubles Model Using Variational Brueckner Orbitals. Application to Symmetry Breaking in O4. 

The Pariser-Parr-Pople Hamiltonian for the description of the 7i-electrons in trans-polyacetylene is reparametrized using ab initio Coupled Cluster Doubles calculations based on a Restricted Hartree Fock reference on trans-butadiene. To avoid the spin contaminations inherent in Unrestricted Hartree Fock (UHF) type calculations on polymethine chains in the doublet state the Annihilated Unrestricted Hartree Fock (AUHF) model is applied in our PPP calculations (tPA (CH) , polyenes H-(CH)2N-H, polymethines H-(CH)2N+1-H). In geometry optimizations on polymethine chains it is shown that in contrast to results from Hiickel type models the width of neutral solitons is strongly... 

In the following, the theory of Kutzelnigg s linear R12 functions shall be presented and analyzed in the framework of the coupled-cluster doubles (CCD) method, To illustrate the ideas and approximations employed in the linear R12 methods, it is sufficient to consider the CCD model, as the corresponding CCD-R12 theory exhibits all properties of the R12 theories. It is a relatively simple matter to include singles (CCSD) or even triples (for example in the CCSD(T) method), and CI-R12-type wave functions or MPn-R12 energies require essentially the same computational procedures as the CCD-R12 approach. 

Since the excitation operators (13.1.2) commute among one another, there are no problems with the order of the operators in this expression. The resulting wave function (13.1.5) corresponds to a particularly simple realization of the coupled-cluster model, in which only double excitations are allowed the coupled-cluster doubles (CCD) wave function. For a complete specification of this model, we must also describe the method by which the amplitudes are determined. We shall... 

Linear Coupled Cluster Doubles (LCCD) model, 139... 

If we limit T to only double excitations, we obtain the coupled-cluster doubles (CCD) model.Equations for the amplitudes can be obtained by back-projecting, H exp(T2) 1 q>, onto the space of Cl double excitations. This leads to a set of nonlinear coupled equations for the amplitudes ... 

However, there is an interesting intermediate step between standard CCD and ECCD. This is a model that includes terms quadratic in A2 in the bra function. We term this energy functional the quadratic coupled cluster doubles (QCCD) method (79) ... 

Christiansen O, Koch H, Jorgensen P (1995) The second-order approximate coupled cluster singles and doubles model CC2. Chem Phys Lett 243 409 t 18... 

Valeev, E.F., Crawford, T.D. Simple coupled-cluster singles and doubles method with perturbative inclusion of triples and explicitly correlated geminals the CCSD(T)ri2 model. J. Chem. Phys. 2008, 128, 244113. 

Kohn, A., Richings, G.W., Tew, D.P. Implementation of the full explicitly correlated coupled-cluster singles and doubles model CCSD-F12 with optimally reduced auxiliary basis dependence. J. Chem. Phys. 2008, 129, 201103. 

CCSD Model. Coupled Cluster Singles and Doubles. A Correlated Model. 

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