The use of bond centred fitting functions has a dual purpose. Firstly it allows better modelling of the charge distribution in a bond centre, which is normally where the valence charge is localised in a cluster. Secondly it removes the need to include d- orbital fitting functions on the bulk Si atoms. This is because the bond centred Gaussians act on their nearby atomic sites with s, p, d, etc. character, the strength of this effect tailing off with distance between the Gaussian and the atomic site and increasing quantum number. In order to maximise d-like character it is possible to arrange these fitting functions symmetrically around an atomic site so that their p-character exactly cancels; the minimum number of functions required for this is an icosohedral arrangement. We tried this for some of the oxygen diffusion calculations, where bonds are breaking and reforming and so it is not always obvious where bond centred functions should be located (see Chapter 6), however in practise they did not increase the accuracy of the calculation and significantly slowed calculation time.
However bond centred Gaussian functions bring associated disadvantages. Such additional fitting functions will locally improve the wavefunction and charge density modelling, thus lowering the cluster energy. This makes it harder to compare different structures, where the energy may change simply because the bond centred functions are no longer in such optimal locations. This has proved to be a particular problem in the case of oxygen in silicon where the cluster energy is critically dependant on the number and location of bond centred fitting functions. They often have little effect on final structure but the improved local charge description can improve the local vibrational modes.