Correlation coupled-clusters

If we compare results obtained with the same basis sets with the three coupled cluster models CCS, CC2 and CCSD, we find similar trends as observed in Refs. [22,45] The CCS model underestimates strongly the static hyperpolarizabilities and their dispersion. The results are usually of similar quality as those obtained with SCF. For methane, the CCS static hyperpolarizabilities are intermediate between the SCF and the CCSD values obtained in the same basis set. In Ref. [45] the CCS percentage dispersion contribution to the third harmonic generation (THG) hyperpolarizability of methane was found to be slightly smaller than for SCF, both underestimating significantly the dispersion obtained with the correlated coupled cluster models CC2 and CCSD. Accordingly the CCS dispersion coeflBcients listed in Table 3 are substantially smaller than the respective CCSD results obtained in the same basis sets. 

Keywords explicit-r12 correlation coupled-cluster methods higher-order excitations... 

Shiozaki, T., Kamiya, M., Hirata, S., Valeev, E.F. Equations of explicitly-correlated coupled-cluster methods. Phys. Chem. Chem. Phys. 2008, 10, 3358-70. 

Neiss, C., Hattig, C. Frequency-dependent nonlinear optical properties with explicitly correlated coupled-cluster response theory using the CCSD(R12) model. J. Chem. Phys. 2007, 126, 154101. 

Noga, J., Valiron, P., Klopper, W. The accuracy of atomization energies from explicitly correlated coupled-cluster calculations. J. Chem. Phys. 2001, 115, 2022-32. 

Noga, J., Kedzuch, S., Simunek, J., Ten-no, S. Explicitly correlated coupled cluster F12 theory with single and double excitations. J. Chem. Phys. 2008, 128, 174103. 

Tew, D.P., Klopper, W., Hattig, C. A diagonal orbital-invariant explicitly-correlated coupled-cluster method. Chem. Phys. Lett. 2008, 452, 326-32. 

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. 

Explicitly Correlated Coupled Cluster R12 Calculations (J. Noga P. Valiron)... 

Performance of Block Correlated Coupled Cluster Method with the CASSCF Reference Function for Carbon-Carbon Bond Breaking in Hydrocarbons... 

Abstract The block correlated coupled cluster method, with the complete active-space self-consistent-field reference function (CAS-BCCC), has been applied to investigate the bond-breaking potential energy surfaces (PESs) for a C—C bond in two alkanes (ethane and 2,3-dimethyl-butane) and a C=C bond in two alkenes (ethylene and 2,3-dimethyl-2-butene). The results are compared with those from other multireference methods (CASPT2, MR-CISD, and MR-CISD+Q). It is demonstrated that the CAS-BCCC method can provide more accurate PESs for C—C bond-breaking PESs than CASPT2 and MR-CISD. The overall performance of CAS-BCCC is shown to be comparable to that of MR-CISD-pQ for systems under study. 

Keywords Block correlated coupled cluster method energy surface Size-extensivity error... 

In our previous work [96], we have developed an alternative multireference method, block correlated coupled cluster (BCCC) approach, for the electronic structure calculation of systems with noticeable multireference character. In this approach, the reference function is expressed as the tensor product of the most important many-electron states in each block, assuming that all orbitals in a system... 

Performance of Block Correlated Coupled Cluster Method... 

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