New Method for Quantitative Electronic Structure Calculation

新的碳基材料，如富勒烯和纳米管，可以采取各种各样的拓扑结构，从而具有各种各样的物理性质。因此，作为新型功能材料和器件材料，它们不仅在科学领域，而且在技术领域都受到高度关注。另一方面，由于它们复杂的网络结构以及它们的大单元尺寸，这些有趣材料的第一性原理电子结构研究通常非常昂贵。因此，更为简化的量子力学方法——紧束缚法，正在引起人们的重新关注。已经建立了一个定量可靠的碳紧密结合模型。然而，在以富勒烯为基础的离子化合物中，每个富勒烯上多余电子之间的远距离库仑相互作用（马德隆能）和库仑斥力应该是非常重要的，但通常的紧密结合模型中没有包括。我们制定了紧密约束模型，其中适当地纳入了上述两个重要术语。将新的紧密结合模型应用于聚合物k_1，发现中等振幅电荷密度波态是可能的稳定态。目前对紧密结合方法的改进已被证明是有效的。同时指出了包含自旋极化这一紧密结合模型的未来发展方向。

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.