Ab.initio method of quantum chemistry
The ab.initio methods of quantum chemistry are methods of numerical Chimie based on the quantum Chimie.
The method ab.initio simplest of electronic structural analysis is the diagram Hartree-Fock (HF), in which the Coulomb repulsion electron-electron is not specifically taken into account. Only its average effect is included in calculation. When the size of the base is increased, the energy and the function of wave tighten towards a limit called limit Hartree-Fock . Many types of calculations, known under the name of methods Post-Hartree-Fock, start with a Hartree-Fock calculation and are then corrected for the repulsion electron-election, so known like electronic Corrélation. The Perturbation theory of Møller-Plesset (MP N ) and the theory of the clusters coupled (DC) are examples of these methods Post-Hartree-Fock ,
In certain cases, particularly for the processes of ruptures of connections, the Hartree-Fock method is inadequate and this function of reference to only one determinant is not a good base for the methods post-Hartree-Fock. It is then necessary to begin with a function from wave including/understanding more than one determinant like a self-coherent multi-configurational Champ and methods were developed using these references multi-determinants for improvements.
In the diagram of smoothing of density , the Integral with four indices used to describe the interaction between the pairs of electrons are reduced to integrals with two or three simpler indices, by treating the densities of loads which they contain in a simplified way. That reduces the scalability compared to the size of the bases. The methods employing this diagram are indicated by the prefix {guil|df-}} , thus for example MP2 with smoothing of density is df-MP2 (the small letters are indicated to avoid confusion with DFT). In the approximation of the local orbital , the orbital molecular ones, which is formally extended to the whole molecule, is restricted with restricted fields. That eliminates the interactions between the pairs from distant electrons and thus reduce drastiquement scalabilté compared to the molecular size, an main issue in the treatment of the molecules to biological size. The methods employing this diagram are indicated by the prefix
L
, e.g. LMP2. These two diagrams can be employed together, as in the method df-LMP2 recently developed.