Density-functional theory (DFT) in the realization of Kohn and Shan (KS) [1], and within the local-density approximation (LDA) - partially improved by gradient correction (GC) techniques - has become well established in studies of the electronic structure and structure of molecules, clusters and solids. DFT-LDA-(GC) calculations allow the study of rather large systems with a reasonable computational effort and a quite good accuracy of the results. Furtheromore, the DFT-LDA, allows also the study of dynamical processes through the coupling with molecular dynamics [2]. However, there are still many systems that are too large to be studied by full "ab initio" techniques, as for example large clusters, biomolecules or solids with very large unit cells or even amorphous solids. Moreover, to the study of the dynamics of complex systems over a long simulation time requires approximate schemes.
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