Examples of 4-Component Basis Sets in Applications

We now turn to a few specific examples of 4-coraponent basis sets and their applications. It should be realized that the derivation and testing of relativistic basis sets has not been a very systematic exercise. This is due to the fact that the calculations themselves are demanding, and in particular for the heavy elements, where the relativistic efiects are likely to be most noticable. Thus many basis sets have been developed and applied only for specific applications on one, or a limited number of molecules. Here we will review mostly those developments that have a direct bearing on the present state of molecular 4-component calculations. 

Early work was partly carried out using non-relativistic basis sets either unmodified, or augmented. However, the need to explore specially adapted basis sets quickly became apparent. At first sets of exponents were optimized by fitting the to numerical atomic DHF functions, as in the work 

Energies and basis set sizes for the heavier noble gases. AE is the difference between the numerical DHF result and the finite basis result. The number next to the noble gas symbol is the atomic mass used to parametrize the finite nuclear size. 

Basis set studies for the bond length of TlAt. is the change 

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