Main Group Inorganics, Noble Gases, and Alkali Metals

2 MAIN GROUP INORGANICS, NOBLE GASES, AND ALKALI METALS 

Modeling the elements discussed in this section is fairly similar to modeling organic compounds. This is primarily because d and/orbitals play a minor role in their chemistry. When d and/ orbitals do affect the chemistry, their effect is well defined and for the most part understood. 

Molecular mechanics methods have only been used to a limited extent for these classes of compounds. However, molecular mechanics methods do fairly well in describing the geometries and relative energies of compounds with these elements. It is perhaps only for historical and economic reasons that molecular mechanics has not been used more for modeling these elements. Subsequently, there are not as many force fields available. 

Semiempirical, DFT, and ah initio methods also work well. Correlation effects are sometimes included for the sake of increased accuracy, but are not always necessary. One particular case for which correlation is often necessary is fluorine compounds. 

Modeling the lighter main group inorganic compounds is similar to modeling organic compounds. Thus, the choice of method and basis set is nearly identical. The second-row compounds (i.e., sulfur) do have unfilled d orbitals, making it often necessary to use basis sets with d functions. 

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