Gaussian basis functions electron correlation effects

The main problem with the DFT methods is the lack of a recipe to find the appropriate forms of the <, and functionals. On the other hand, the main problem with the ab initio methods is the computation of the electronic correlation effects neglected in the HF approximation. All the pos-HF methods shown in Table VILl exhibit a rather steep increase in computational cost as the size of the basis set increases. In order to obtain a good estimate of the electronic correlation effects without making use of very large basis sets, calculations from different levels of theory can be combined. Examples of such strategy are the complete basis set (CBS) family of methods, developed by Petersson et al. [21] and the Gaussian (Gn) methods, developed by Pople and co-workers [22], By choosing the appropriate level of theory and size of the basis set to be used at each different step of the calculation, the CBS and Gn methods can produce total... 

The symmetry requirements and the need to very effectively describe the correlation effects have been the main motivations that have turned our attention to explicitly correlated Gaussian functions as the choice for the basis set in the atomic and molecular non-BO calculations. These functions have been used previously in Born-Oppenheimer calculations to describe the electron correlation in molecular systems using the perturbation theory approach [35 2], While in those calculations, Gaussian pair functions (geminals), each dependent only on a single interelectron distance in the exponential factor, exp( pr ), were used, in the non-BO calculations each basis function needs to depend on distances between aU pairs of particles forming the system. 

The Ni atoms are treated as one-electron systems in which the effects of the Ar-like core and the nine 3d electrons are replaced by a modified effective potential (MEP) as suggested by Melius et al./165/ A contracted gaussian basis set is used for Ni, which includes two functions to describe the 4s and one to describe the 4p atomic orbitals. Since a previous study/159/ found the O-Ni spacing and the vibrational frequency we insensitive to correlations in this open-shell system, the authors have adopted the SCF calculation scheme. To check the approximation of treating the Ni atoms as a one-electron system, they performed both the MEP and an all electron SCF calculation for the NisO cluster. They found that the MEP spacing is 0.37 A or 35% smaller than the all-electron value the MEP we value is 90 cm-1 or 24% smaller. Since the 3d... 

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