Quadratic configuration interaction method

Coupled Cluster and Quadratic Configuration Interaction Methods... 

The concomitant advances in theoretical methodologies and algorithms have also played a vital role in increasing computational capabilities for theoretical thermochemistry. These advances include (1) new methods for accurate treatment of electron correlation in molecules and atoms such as coupled cluster and quadratic configuration interaction methods, (2) new basis sets such as the correlation consistent basis sets, and (3) development of model chemistry ... 

Coupled-cluster and quadratic configuration interaction methods... 

An extensive quantum-mechanical study that included correlation by second-order Meller-Plesset perturbation and quadratic configuration-interaction methods showed that the cluster species (NH4)2 is bound with respect to 2 NH4 by 7.5 to 9.7 kcal/mol, but unstable with respect to 2 NH3 + H2 by 86 to 89 kcal/mol. The bonding results from the interaction of the 3sai Rydberg orbitals of NH4. A minimum-energy structure of C2h symmetry was predicted. 

Gauss, J. and Cremer, D. (1992). Analytical energy gradients in M0ller-Plesset perturbation and quadratic configuration interaction methods theory and application. Adv. Quantum Chem., 23, 205-299. 

CBS extrapolation procedure Coupled cluster methods Exphcitly correlated CCSD(T) computation Configuration interaction methods and quadratic configuration interaction methods Counterpoise-correction procedure... 

Among the most widely used ab initio methods are those referred to as Gl" and 02." These methods incorporate large basis sets including d and / orbitals, called 6-311. The calculations also have extensive configuration interaction terms at the Moller-Plesset fourth order (MP4) and fiirther terms referred to as quadratic configuration interaction (QCISD). ° Finally, there are systematically applied correction terms calibrated by exact energies from small molecules. 

A disadvantage of all these limited Cl variants is that they are not size-consistent.The Quadratic Configuration Interaction (QCI) method was developed to correct this deficiency. The QCISD method adds terms to CISD to restore size consistency. QCISD also accounts for some correlation effects to infinite order. QCISD(T) adds triple substitutions to QCISD, providing even greater accuracy. Similarly, QCISD(TQ) adds both triples and quadruples from the full Cl expansion to QCISD. 

Curtiss, L. A. Raghavachari, K. Pople, J. A. Gaussian-2 theory use of higher level correlation methods, quadratic configuration interaction geometries, and second-order Mpller Plesset zero-point energies. J. Chem. Phys. 1995, 103, 4192-4120. 

There is also a hierarchy of electron correlation procedures. The Hartree-Fock (HF) approximation neglects correlation of electrons with antiparallel spins. Increasing levels of accuracy of electron correlation treatment are achieved by Mpller-Plesset perturbation theory truncated at the second (MP2), third (MP3), or fourth (MP4) order. Further inclusion of electron correlation is achieved by methods such as quadratic configuration interaction with single, double, and (perturbatively calculated) triple excitations [QCISD(T)], and by the analogous coupled cluster theory [CCSD(T)] [8],... 

G. A. Petersson, T. G. Tensfeldt, and J. A. Montgomery Jr.,/. Chem. Phys., 94,6091 (1991). A Complete Basis Set Model Chemistry. III. The Complete Basis Set-Quadratic Configuration Interaction Family of Methods. 

L. A. Curtiss, K. Raghavachari, and J. A. Pople, /. Chem. Phys., 103, 4192 (1995). Gaus-sian-2 Theory Use of Higher Level Correlation Methods, Quadratic Configuration Interaction Geometries, and Second-Order Moller-Plesset Zero-Point Energies. 

In the 1980s, Pople and co-workers developed the non-variational quadratic configuration interaction (QCI) method, which is intermediate between CC and Cl methods. Similar to the CC methods, QCI also has the corresponding QCISD and QCISD(T) options. Both the CCSD(T) and QCISD(T) have been rated as the most reliable among the currently computationally affordable methods. 

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