Relativistic effective core potentials complete

Unlike semiempirical methods that are formulated to completely neglect the core electrons, ah initio methods must represent all the electrons in some manner. However, for heavy atoms it is desirable to reduce the amount of computation necessary. This is done by replacing the core electrons and their basis functions in the wave function by a potential term in the Hamiltonian. These are called core potentials, elfective core potentials (ECP), or relativistic effective core potentials (RECP). Core potentials must be used along with a valence basis set that was created to accompany them. As well as reducing the computation time, core potentials can include the effects of the relativistic mass defect and spin coupling terms that are significant near the nuclei of heavy atoms. This is often the method of choice for heavy atoms, Rb and up. 

Y. S. Kim, Y. S. Lee. The Kramers restricted complete active space self-consistent-field method for two-component molecular spinors and relativistic effective core potentials including spin-orbit interactions. /. Chem. Phys., 119 (2003) 12169. 

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With the exception of Ligand Field Theory, where the inclusion of atomic spin-orbit coupling is easy, a complete molecular treatment of relativity is difficult although not impossible. The work of Ellis within the Density Functional Theory DVXa framework is notable in this regard [132]. At a less rigorous level, it is relatively straightforward to develop a partial relativistic treatment. The most popular approach is to modify the potential of the core electrons to mimic the potential appropriate to the relativistically treated atom. This represents a specific use of so-called Effective Core Potentials (ECPs). Using ECPs reduces the numbers of electrons to be included explicitly in the calculation and hence reduces the execution time. Relativistic ECPs within the Hartree-Fock approximation [133] are available for all three transition series. A comparable frozen core approximation [134] scheme has been adopted for... 

Let us consider the principal results of these calculations in the case of lanthanum trichloride, for which the most complete ab initio and density fimctional theory (DFT) calculations (both with various basis set modifications) are available. The obtained molecular parameters strongly depend on the valence basis sets and effective relativistic core potentials. Nevertheless, ab initio calculations tend to predict a planar structure, whereas DFT calculations favor pyramidal configurations. The V2 frequencies obtained in all calculations are too low compared with those found experimentally. This implies a low barrier to inversion and large thermal geometry fluctuations. In view of such uncertainty in the results of quantum-chemical calculations, Hargittai (2000) was very careful in her estimates. She tends to favor the planar geometry of lanthanide chlorides, although does not rule out the possibility that they are pyramidal. 

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