Block correlated coupled cluster method

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... 

Performance of Block Correlated Coupled Cluster Method... 

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... 

Today we know that the HF method gives a very precise description of the electronic structure for most closed-shell molecules in their ground electronic state. The molecular structure and physical properties can be computed with only small errors. The electron density is well described. The HF wave function is also used as a reference in treatments of electron correlation, such as perturbation theory (MP2), configuration interaction (Cl), coupled-cluster (CC) theory, etc. Many semi-empirical procedures, such as CNDO, INDO, the Pariser-Parr-Pople method for rr-eleetron systems, ete. are based on the HF method. Density functional theory (DFT) can be considered as HF theory that includes a semiempirical estimate of the correlation error. The HF theory is the basie building block in modern quantum chemistry, and the basic entity in HF theory is the moleeular orbital. 

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