Post-SCF

The HE, GVB, local MP2, and DFT methods are available, as well as local, gradient-corrected, and hybrid density functionals. The GVB-RCI (restricted configuration interaction) method is available to give correlation and correct bond dissociation with a minimum amount of CPU time. There is also a GVB-DFT calculation available, which is a GVB-SCF calculation with a post-SCF DFT calculation. In addition, GVB-MP2 calculations are possible. Geometry optimizations can be performed with constraints. Both quasi-Newton and QST transition structure finding algorithms are available, as well as the SCRF solvation method. 

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. 

Calculations for rutile and anatase were performed using both LDA level of DFT and the gradient corrected form of the exchange-correlation potential (GGA). The GGA approach is implemented self-consistently as described in 18 and not as a post-SCF correction. 

The cc correlation energy is determined by the single and double amplitudes and the two-electron MO integrals. However, the recent progress in computational methods is largely influenced by DFT. One primary advantage of DFT over conventional HF-SCF procedures is that the former includes electron correlation fairly adequately (some times too much) at a fractional cost compared to a typical post-SCF (Cl, MBPT, or CC) calculation. 

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... 

P. Cysewski, Z. Czyznikowska, R. Zalesny, P. Czeleh, The post-SCF quantum chemistry characteristics of guanine-guanine stacking in B-DNA. Phys. Chem. Chem. Phys. 10, 2665-2672 (2008)... 

The first explicit PCM contribution is common to all post-SCF gradients and involves the change in the 1PDM made by the post-SCF procedure [26, 28] ... 

Tolosa, S., Espinosa, J., and Olivares del Valle, F. J., Computation of spectroscopic properties of van der Wards systems from post-SCF ah initio potentials including the EICP alternative counterpoise technique, J. Comput. Chem. 5, 611-619 (1991). 

DISP Dispersion energy. An interaction resembling London forces, present only in post-SCF calculations. 

An important issue with regard to any perturbation treatment is the convergence behavior of the perturbation series. This is considered in Section 4 where problematic cases are identified. Then a potentially viable treatment of such cases, based on vibrational SCF and post-SCF procedures, is elaborated in Section 5. In Section 6 we turn to tire practical Issues of basis set requirements and treatment of electron correlation. Here tire emphasis is on quasilinear pi-conjugated molecules and, for that case, we examine the difficulties encountered with tire use of density functional theory. 

All the calculations have been made with the Gaussian 94 [5] or the Gaussian 98 [6] program. Molecular structures for all molecules studied here have been calculated by geometry optimization within the procedures of the restricted Hartree-Fock method (HF) but good agreement with experimental structures is obtained only accidentally with HF when using small basis sets, so that Post-SCF... 

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. 

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