Hartree-Fock self-consistent field correlation problem

If we except the Density Functional Theory and Coupled Clusters treatments (see, for example, reference [1] and references therein), the Configuration Interaction (Cl) and the Many-Body-Perturbation-Theory (MBPT) [2] approaches are the most widely-used methods to deal with the correlation problem in computational chemistry. The MBPT approach based on an HF-SCF (Hartree-Fock Self-Consistent Field) single reference taking RHF (Restricted Hartree-Fock) [3] or UHF (Unrestricted Hartree-Fock ) orbitals [4-6] has been particularly developed, at various order of perturbation n, leading to the widespread MPw or UMPw treatments when a Moller-Plesset (MP) partition of the electronic Hamiltonian is considered [7]. The implementation of such methods in various codes and the large distribution of some of them as black boxes make the MPn theories a common way for the non-specialist to tentatively include, with more or less relevancy, correlation effects in the calculations. 

With the success of these calculations for isolated molecules, we began a systematic series of supermolecule calculations. As discussed previously, these are ab initio molecular orbital calculations over a cluster of nuclear centers representing two or more molecules. Self-consistent field calculations include all the electrostatic, penetration, exchange, and induction portions of the intermolecular interaction energy, but do not treat the dispersion effects which can be treated by the post Hartree-Fock techniques for electron correlation [91]. The major problems of basis set superposition errors (BSSE) [82] are primarily associated with the calculation of the energy. 

A number of important trends can be drawn from Table 4.1, which are trends that have influenced how computational chemists approach related (and sometimes even largely unrelated) problems. Hartree-Fock (HF) self-consistent field (SCF) computations vastly overestimate the barrier, predicting a barrier twice as large as experiment. The omission of any electron correlation more seriously affects the transition state, where partial bonds require correlation for proper description, than the ground-state reactants. Inclusion of nondynamical correlation is also insufficient to describe this reaction complete active space self-consistent field (CASSCF) computations also overestimate the barrier by some 20 kcal... 

The simplest of the ab initio methods which ignores electron-correlation effects completely is the self-consistent field (SCF) method and is described in sufiScient details in the textbooks. In principle, it treats the many-electron problem as a one-electron equation in the effective field created by the rest of the electrons. The resulting equation, known as the Hartree-Fock equation, is solved iteratively to self-consistency. 

Basis Sets Correlation Consistent Sets Coupled-cluster Theory Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field Density Functional Theory Applications to Transition Metal Problems G2 Theory Metal Complexes MpUer-Plesset Perturbation Theory Transition Metal Chemistry Transition Metals Applications. 

Density functional theory based methods are now a very popular and rather inexpensive alternative to conventional correlated ab initio methods. However, none of the available DFT methods covers the dispersion energy" which limits their use for interactions of biomolecules. An other limitation to the application of DFT procedures in the realm of biomolecules stems from the fact that the charge transfer interactions (which probably play an important role in the "function of biosystems) are mostly strongly overestimated, though the very good performance of some so-called hybrid methods provides a large improvement. For more details concerning DFT techniques see Density Functional Applications Density Functional Theory (DFT), Hartree-Fock (HF), and the Self-consistent Field and Density Functional Theory Applications to Transition Metal Problems. The application of DFT to DNA base pairs is evaluated in Section 3.2.3. 

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