Alkaline earth elements, core-valence

Other complications are associated with the partitioning of the core and valence space, which is a fundamental assumption of effective potential approximations. For instance, for the transition elements, in addition to the outermost s and d subshells, the next inner s and p subshells must also be included in the valence space in order to accurately compute certain properties (54). A related problem occurs in the alkali and alkaline earth elements, involving the outer s and next inner s and p subshells. In this case, however, the difficulties are related to core-valence correlation. Muller et al. (55) have developed semiempirical core polarization treatments for dealing with intershell correlation. Similar techniques have been used in pseudopotential calculations (56). These approaches assume that intershell correlation can be represented by a simple polarization of one shell (core) relative to the electrons in another (valence) and, therefore, the correlation energy adjustment will be... 

The results of this procedure for alkaline and alkaline-earth systems were quite good [186,187], at least for atoms, and pseudopotentials of this type were generated [188] and applied [189] for most of the main group elements. However, due to the limited validity of the frozen-core approximation when going from a medium or highly charged one-valence electron ion to a neutral atom or nearly neutral ion, the approach is bound to fail for most other elements. This is especially the case for transition metals, lanthanides and actinides, where small cores are indispensable for accurate pseudopotentials. More recent calibration studies of alkaline and alkaline earth elements exhibited however, that for accu-... 

The block s, on the left of the Table, contains the alkali and alkaline earth metals. Each atom of these metals possesses an inert gas core and one or two electrons in the s orbital of the valence shell, that is, an external electron configuration ns1 or ns2 where n is the value of the principal quantum number, and also the period number in the Periodic Table. Notice however that He, owing to its general chemical inertness and to the behaviour similarity with the other noble gasses is generally placed at the far right of the Table. The p block contains elements corresponding to electron... 

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