Coupled cluster doubles

Coupled—Pair Functional (ACPF) and Coupled Electron Pair Approximation (CEPA). The simplest form of CEPA, CEPA-0, is also known as Linear Coupled Cluster Doubles (LCCD). 

Coupled Cluster Doubles (CCD) Compared to the number of doubles, there are ijkabc... 

In spite of the method s present utility and popularity, the quantum chemical community was slow to accept coupled cluster theory, perhaps because the earliest researchers in the field used elegant but unfamiliar mathematical tools such as Feynman-like diagrams and second quantization to derive working equations. Nearly 10 years after the essential contributions of Paldus and Cizek, Hurley presented a re-derivation of the coupled cluster doubles (CCD) equa-... 

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. 

Various approximations of Eq. (6), the exact wave function in the coupled-cluster formalism, have been discussed in the chemical literature. In particular, Cizek s coupled-pair many-electron theory (CPMET),5 also referred to as coupled-cluster doubles (CCD) by Bartlett,8 has re-... 

CCDPPA coupled-cluster doubles polarization propagator approximation (Section VII. E),... 

We find that it is convenient to work with the spin-adapted form of the coupled-cluster doubles (CCD) equations. The spin-adapted double excitation operators S (i),i = 1,2, are given, for example, in Oddershede et al. (1984, Appendix C). 

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... 

The ab initio HF calculations reported below have been performed with the GAUSSIAN 76 [26] program package. The atomic basis sets applied are a minimal (STO-3G [26]) one, a split valence (6-31G [26]) one, a split-valence one plus a set of five d-functions on carbon (6-31G [26]), and one with an additional set of p-functions on hydrogen (6-31G [26]). The correlation energy has been computed using Mpller-Plesset many body perturbation theory of second order (MP2) [27], the linear approximation of Coupled Cluster Doubles theory (L-CCD)... 

Inclusion of only J2 gives an approximate CC approach called the coupled-cluster doubles (CCD) method. Since = + T2 + lf + , the wave function i ccd contains determinants with double substitutions, quadruple substitutions, hextuple substitutions, and so on. Recall (Section 15.17) that quadruple substitutions are next in importance after double substitutions in a Cl wave function. The treatment of quadruple substitutions in the CCD method is only approximate. The CCD quadruple excitations are produced by the operator tl, and so the coefficients of the quadruply substituted determinants are determined as products of the coefficients of the doubly substituted determinants [see Eq. (15.92)], rather than being determined independently, as in the CI-SDTQ method The CCD approximation of the coefficients of the quadruply substituted determinants turns out to be pretty accurate. 

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