Post-SCF methods

A variety of theoretical methods have been developed which include some effects of electron correlation. Traditionally, such methods are referred to as post-SCF methods because they add correlation corrections to the basic Hartree-Fock model. As of this writing, there are many correlation methods available in Gaussian, including the following ... 

Methods based on Density Functional Theory also include some electron correlation effects (we ll consider them a bit later in this chapter). Of the traditional post-SCF methods, we ll be primarily using MP2, MP4, QCISD and QCISDfO in this work. 

Density-Functional Theory. Transition metals pose a problem for classical quantum chemical methods like self-consistent field (SCF), perturbation theory, configuration interaction (Cl), and variations on these methods, because of the very large electron correlation. SCF underestimates binding substantially, and post-SCF methods are so expensive for transition metals that one can do a calculation only on models with few atoms. DFT on the other hand is relatively cheap it is about as expensive as SCF. Moreover, with the development of the generalized-gradient approximations it is also reasonably accurate. A large majority of quantum chemical... 

Any method which goes beyond SCF in attempting to treat this phenomenon properly is known as an electron correlation method (despite the fact that Hartree-Fock theory does include some correlation effects) or a post-SCF method. We will look briefly at two different approaches to the electron correlation problem in this section. 

The efforts to extend the direct SCF methodology to post-SCF calculations began with the paper by Taylor.Soon afterward, practical implementations of the direct MP2 method were published, almost simultaneously, by Pople s and Almlof s groups. The latter report also considered the direct MP3 method and presented an example of an MP2 calculation with 318 basis functions. The direct post-SCF methods are discussed later in this review. 

A more economical alternative to Cl for determining the coefficients Cj in Eq. [4] is provided by methods based on many-body perturbation theory (MBPT). The most widely used variant of MBPT is the Moller-Plesset (MP) scheme. For reasons described in the section on post-SCF methods, MP calculations are less computationally demanding, bur also less accurate, than Cl calculations involving the same set of excited configurations. 

POST-SCF METHODS HOW TO AVOID BUILDING CASTLES ON SAND... 

DFT+ALL method is advantageous over the other post HF methods such as MP2, since the present method is applicable to larger systems, which cannot be calculated by the post SCF methods. 

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