Spectroscopy of embedded molecules

Numerous crystals doped by ionic impurities such as main element ions or / element ions offer very attractive photophysical properties. One of the key questions is to identify the factors that make a host-impurity combination efficient. This requires the knowledge of energy levels. In many cases, the energy scheme of the electronic levels can be nicely imderstood by taking into account the proper electronic and crystal field interactions in standard crystal-field theory [161], that uses some effective parameters extracted from experimental data. However, ab initio methods are interesting tools as they ideally aim at studying and understanding physical phenomena, at the atomic scale. They are only based on first principles and fundamental constants, and can therefore. 

The following sections review a selection of characteristic recent applications of ab initio methods to the spectroscopy of ionic impurities. Our main goal is to illustrate the strengths and limitations of existing approaches to treat electron correlation and spin-orbit effects, and we will also emphasise the importance of crystal effects. 

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