Hartree Fock many-body forces

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. 

The ab initio method used to obtain the potential surface employs many-body perturbation theory (MBPT) relative to an unrestricted Hartree-Fock (UHF) reference function. Correlation is treated to fourth order in MBPT and includes contributions from all single, double, and quadruple excitations. A Gaussian basis of double zeta plus polarization functions (DZP) is used. This model is computationally efficient and has been shown to provide accurate force fields and geometries and reliable thermochemical data for a wide variety of molecular systems.It has also been used to calculate reaction paths for some triatomic systems. 

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