Small molecules electron-correlated calculations

Gauss has derived shielding theory with electron correlated calculations in the Moller-Plesset expansion (43,44) and also in the coupled cluster approach (45). He has provided benchmark calculations on a set of small molecules shown in Tables II and III at the CCSD(T) level (46). 

Styszyriski et al. [31] studied the performance of BFs in relativistic HF and non-relativistic and relativistic electron correlation calculations for the BF, AIF, and GaF molecules. It was found that at both the non-relativistic and relativistic HF levels, the diffuse and polarization functions of an AC basis set can be effectively substituted by BFs unless the calculations with the AC basis itself yield near-HF-limit energies. However, in correlation calculations, the benefits of BFs were not obvious because when many BFs were used, the rate of convergence was usually rained. Nevertheless, when the number of BFs was relatively small, a considerable improvement could be observed in the correlation energy at practically no additional computational cost. 

Group 2 complexes are formally electron deficient and conformationally floppy only small energies (often only 1-2 kcal mol-1) are required to alter their geometries by large amounts (e.g., bond angles by 20° or more). In such cases, the inclusion of electron-correlation effects becomes critical to an accurate description of the molecules structures. Both HF/MP2 and density functional theory (DFT) methods have been applied to organoalkaline earth compounds. DFT approaches, which implicitly incorporate electron correlation in a computationally efficient form, are generally the more widely used. Molecular orbital calculations that successfully reproduce bent... 

During the last decade MO-theory became by far the most well developed quantum mechanical method for numerical calculations on molecules. Small molecules, mainly diatomics, or highly symmetric structures were treated most accurately. Now applicability and limitations of the independent particle, or Hartree-Fock (H. F.), approximation in calculations of molecular properties are well understood. An impressive number of molecular calculations including electron correlation is available today. Around the equilibrium geometries of molecules, electron-pair theories were found to be the most economical for actual calculations of correlation effects ). Unfortunately, accurate calculations as mentioned above are beyond the present computational possibilities for larger molecular structures. Therefore approximations have to be introduced in the investigation of problems of chemical interest. Consequently the reliability of calculated results has to be checked carefully for every kind of application. Three types of approximations are of interest in connection with this article. 

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