Step 3 Development of Nanoparticle Potentials

When a cluster or nanoparticle becomes larger, a first-principles calculation becomes very expensive and prohibitive. Provided one already has reliable potentials from small and medium-size nanoparticles, it will be possible to construct independent potentials for large cluster or nanoparticle. The resultant potential can be directly compared with empirical models (Step 1). From the comparison, one can refine the experimental models. Such comparison could be repeated until the model potential is optimized. Another direct consequence from this comparison is that it helps one to identify some areas for further theoretical developments. Experimentally, features such as impurities, structural disorders, grain boundaries and oxidation may be present in real samples. Theoretically, depending on the level of sophistication -self-consistent field, configuration interaction, multi-reference self-consistent field and multi-reference configuration interaction - the present first-principles calculations may give different potential surfaces. This is especially true when the compounds have transition metal elements, as often is the case in biological molecules, where the electron correlation is strong. Here, perturbation theory breaks down, although one may be able to develop a very accurate density-matrix renormalization group technique for model systems [30-32]. The initial results are quite promising, where one sees a clear improvement over traditional quantum chemistry methods. The main idea of this method is to retain the most relevant information while properly treating the boundary condition. This method is probably one of the most accurate methods for treating large systems. With the improved quality of theoretical calculations and model simulations, one should be ready to predict and design new materials with desirable features.

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