Quadratic configuration interaction QCI

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. 

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. 

Two approaches to electron correlation that are widely used today for the studies of organic radical cations are Coupled Cluster (CC) calculations or the similar, but not identical, Quadratic Configuration Interaction (QCI) method with single and double excitations, often followed by CCSD(T) or QCISD(T) single point calculations with a larger basis set. These methods suffer to a much lesser extent from... 

Accounting for relativistic effects in computational organotin studies becomes complicated, because Hartree-Fock (HF), density functional theory (DFT), and post-HF methods such as n-th order Mpller-Plesset perturbation (MPn), coupled cluster (CC), and quadratic configuration interaction (QCI) methods are non-relativistic. Relativistic effects can be incorporated in quantum chemical methods with Dirac-Hartree-Fock theory, which is based on the four-component Dirac equation. " Unformnately the four-component Flamiltonian in the all-electron relativistic Dirac-Fock method makes calculations time consuming, with calculations becoming 100 times more expensive. The four-component Dirac equation can be approximated by a two-component form, as seen in the Douglas-Kroll (DK) Hamiltonian or by the zero-order regular approximation To address the electron cor-... 

Pople and co-workers developed the nonvariational quadratic configuration-interaction (QCI) method, which is intermediate between the CC and Cl methods. The QCI method exists in the size-consistent forms QCISD, which is an approximation to CCSD, and QCISD(T), which is similar to CCSD + T(CCSD). QCISD(T) has given excellent results for correlation energies in many calculations and is available as an option in Gaussian [J. A. Pople, M. Head-Gordon, and K. Raghavachari, J. Chem. Phys., 87, 5968 (1987) 90, 4635 (1989) K. Raghavachari and G. W. Trucks, /. Chem. Phys., 91, 1062, 2457 (1989) J. Paldus et al., J. Chem. Phys., 90, 4356 (1989)]. 

The total molecular energy of the ground state was estimated to be Ef P=-342.648 E [4, 5] the so far best theoretical value, E = -341.42843 Eh, resulted from a recent G2 calculation using a method that treats the electron correlation by Moller-Plesset perturbation theory (MP4) and quadratic configuration interaction (QCI) [6]. 

Although a wide variety of theoretical methods is available to study weak noncovalent interactions such as hydrogen bonding or dispersion forces between molecules (and/or atoms), this chapter focuses on size consistent electronic structure techniques likely to be employed by researchers new to the field of computational chemistry. Not stuprisingly, the list of popular electronic structure techniques includes the self-consistent field (SCF) Hartree-Fock method as well as popular implementations of density functional theory (DFT). However, correlated wave function theory (WFT) methods are often required to obtain accmate structures and energetics for weakly bound clusters, and the most useful of these WFT techniques tend to be based on many-body perturbation theory (MBPT) (specifically, Moller-Plesset perturbation theory), quadratic configuration interaction (QCI) theory, and coupled-cluster (CC) theory. 

One popular modification of the standard coupled-cluster model is the quadratic configuration-interaction (QCI) model, originally introduced as a size-extensive amendment of the Cl model [33]. We here discuss the QCI singles-and-doubles (QCISD) model within the framework of similarity-transformed (linked) coupled-cluster theory, from which it is obtained by omitting certain commutators in the CCSD equations. Expanding the remaining commutators, we then go on to express the QCISD equations in a form that illustrates its historical connection to CISD theory. 

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