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The methodsdescribed in this section enable us to perform accurate and reliable calculations of the properties of perfect and defective lattices where good potential models of the pair-wise type may be derived. The limitation of the type of potential model which may be effectively used in the calculations has tended to confine their application to the study of ionic and semi-ionic materials; the types...
It is clear that one of the weakest points of current theories of point defects is in the treatment of the electronic structure of the host lattice. One of the advantages of cluster calculations is that the defect and lattice can be treated together. Great care is needed to incorporate the electrostatic field of the lattice outside the cluster correctly. Polarisation and distortion can be handled...
The following points have been stressed in this brief survey:(i)The ground-state energy E is very naturally considered as E[ϱ(r{{ℓ}}], p being the electron density and {ie113-1} the totality of nuclear sites.(ii)Whereas the wave equation gives a delocalized ϱ(r{{ℓ}}) inevitably (and only determines localized distributions uniquely in a special case like NaCl), physical and chemical intuition can go...
In conclusion then, this brief survey has sought to bring together just some of the diverse problems that are now tractable from a computational point of view. In the forseeable future two areas stand out as ripe for further progress. They are:(i)The development of improved solid state potentials, including many body effects and refinements to the shell model(ii)The application of defect lattice methods...
Computer modelling of solids can now clearly handle complex problems; examples are given by the superionic and mineral systems described earlier in this paper. Complex systems can also be tackled, of which the minerals discussed above provide a good illustration, Future progress will depend first on improvements in potential models to include more sophisticated features, e.g. three-body terms, in...
We believe that the calculations summarised in this chapter show that a considerable amount of information emerges from the new approach which we have developed for the treatment of silicates. The more extended calculations are at a preliminary stage, but the initial studies are encouraging and the comparisons that we have made suggest that we are able to calculate repulsion energies using a parameter...
We have combined defect and lattice energy calculations in a study of the aggregation of impurity-vacancy dipoles and of Suzuki phase precipitation in alkalihalide crystals. The relative stabilities of nearest neighbour compared to next-nearest neighbour configurations of isolated dipoles is accentuated as the cluster size increases. Thus, aggregation is sensitive to the type of dipole which predominates...
This chapter has described the application of computer simulations to the study of crystal surfaces. The methods are general and can be applied to surfaces of any structure. Straightforward extensions of the techniques enable many two-dimensional defects and interfaces to be studied, including stacking faults, grain boundaries, shear planes and interfaces between different materials. Clearly, more...
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