Orthogonalization procedures in the theory of colour centre hyperfine interactions

Abstract

In many colour centre calculations, a true wavefunction must be obtained from an envelope function or pseudo-wavefunction. The author compares four levels of approximation for such calculations. The true wavefunctions are used to calculate contact hyperfine interactions of model centres in LiCl, KCl, RbF, as well as the F+ centre in BeO and neutral interstitial hydrogen in CaF2. It is shown that commonly used approximations can be very inaccurate, and in particular that inclusion only of first-order terms in the overlap between core orbitals can yield erroneous results. Selected tables of core overlaps are included in the text. An exact core orthogonalization procedure not only corrects a tendency to point-ion pseudo-wavefunctions for F centres to overestimate spin densities on the nearest neighbours, but also seem relatively insensitive to the choice of core wavefunctions.