Geometries and Energies of Point Defects

As an introduction to the theory as it relates to these defect complexes, we point out that the most conspicuous experimental feature of a light impurity such as hydrogen is its high local-mode frequency (Cardona, 1983). Therefore, it is essential that the computational scheme produce total energies with respect to atomic coordinates and, in particular, vibrational frequencies, so that contact with experiment can be established. With total-energy capabilities, equilibrium geometries and migration and reorientation barriers can be predicted as well. 

In this section, we discuss theoretical methods, which can be applied for calculations of photoabsorption and PL spectra of silica and germania nanoparticles. We start with the choice of model cluster simulating these materials and point defects in them and consider methods for geometry optimization in the ground and excited electronic states (Subsection 2.1). This is followed by the description of more advanced quantum chemical methods for accurate calculations of excitation energies (Subsection 2.2) and the section is completed by the discussion on the theoretical procedure used for predicting vibronic spectra associated with point defects (Subsection 2.3). 

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