Coupled-cluster approximation

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... 

Cauchy Moments of Ne, Ar, and Kr Atoms Calculated Using the Approximate Coupled Cluster... 

Again, the coefficients describing the time dependence of the approximate coupled cluster wavefunctions [Pg.190]

The CASSCF/CASPT2 calculations were performed with an active space including the five nd, the (n + l)s, the three (n+ l)p orbitals, and a second set of nd orbitals to account for the double shell effect. The importance of including a second 3d shell in the active space was detected in an early study of the electronic spectrum of the nickel atom [2]. This had already been suggested from MRCI results [1]. The results obtained by RT at about the same time indicated that such effects are effectively accounted for when a method is used that includes cluster corrections to all orders, like the QCI method used by them [3]. This result will hold true also for the less approximate coupled cluster method CCSD(T). 

O. Christiansen, H. Koch, and P. Jorgensen, Chem. Phys. Lett., 243, 409 (1995). The Second-Order Approximate Coupled Cluster Singles and Doubles Model CC2. 

Approximate Coupled Cluster Methods Combined Reduced Multireference and Almost-Linear Coupled Cluster Methods with Singles and Doubles ... 

These cluster results are in remarkably good agreement with the periodic calculations of Jaffe and Hess [130] for the adsorption of CO on the non-polar ZnO(lO-lO) surface,both at the SCF and the correlated levels (Table 5). The Zn atoms at this surface are threefold coordinated as are those on the terraces of the ZnO(OOOl) surface plane. Therefore, the adsorption of CO at the Zn Q atom of the free Zn404 cluster can be taken as a model for the adsorption on the Zn atoms in both surfaces. The good agreement is quite surprising since completely different methods have been used periodic approach vs free cluster at the SCF level, perturbation theory (MP2) vs approximate coupled cluster approach (MC-CEPA) at the correlated level. This is also an indication that both treatments will converge to similar results, provided that the calculations are performed correctly. 

Pedersen, T. B., Sanchez de Meras, A. M.and Koch, H. (2004). Polarizability and optical rotation calculated from the approximate coupled cluster singles and doubles CC2 linear response theory using Cholesky decompositions, /. Chem. Phys. 120, pp. 8887-8897, doiilO.1063/1. 1705575. 

Photoisomerization of gas-phase azobenzene was theoretically calculated by A-self-consistent-field (ASCF) density-functional theory (DFT), time-dependent DFT (TD-DFT), and approximate coupled cluster singles and doubles (R1-CC2) methods and discussed ... 

Refs. [44, 46, 49, 51], the second- and third-order approximate coupled cluster (CC2 and CC3) calculations of Ref. [50], the equation of motion coupled cluster with single, double and perturbative triple excitations calculations of Ref. [45], the symmetry adapted cluster configuration interaction calculations of Ref. [48] and the time-dependent density functional theory calculations using the B3LYP functional of Ref. [39]. 

Order Approximate Coupled Cluster Singles and Doubles Model CC2. 

Christiansen, O., Koch, H., and Jprgensen, P. (19956). The second-order approximate coupled cluster singles and doubles model CC2. Chem. Phys. Lett., 243, 409-418. Christiansen, O., Koch, H., and Jprgensen, P. (1996). Perturbative triple excitation corrections to coupled cluster singles and doubles excitation energies. J. Chem. Phys., 105, 1451-1459. 

Pawlowski, R, Jorgensen, R, H ttig, C. (2004). Gauge invariance of oscillator strengths in the approximate coupled cluster triples model CC3. Chemical Physics Letters, 389, 413. 

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