New Method for Quantitative Electronic Structure Calculation

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New carbon-based materials such as fullerenes and nanotubes can take a variety of topological structures and consequently, have a variety of physical properties. Hence, they are now of high interest not only in scientific fields but also in technological fields as new functional materials and device materials. On the other hand, due to their complicated network structure as well as their large unit-cell size, the first-principles electronic structure study of these interesting materials are often very expensive. Therefore, more simplified quantum mechanical approach, tight-binding method, is now attracting a renewed interest. A quantitatively reliable tight-binding model for carbon has been already constructed. However, in the fullerene-based ionic compounds, the long-range Coulomb interaction (Madelung energy) as well as the Coulomb repulsion between excess electron on each fullerene should be very important but is not included in a usual tight-binding model. We have formulated the tight-binding model in which the above two important terms are properly incorported. The new tight-binding model has been applied to the polymerized K_1Cthe moderate-amplitude charge-density wave state is found to be a possible stable state. The present improvement of the tight-binding method has been proven to be effective. A promising future direction of the tight-binding model, the inclusion of the spin polarization, is also pointed out.