Ching Wai-Yim, Rulis Paul
Department of Physics, University of Missouri-Kansas City, Kansas City, MO 64110, USA.
J Phys Condens Matter. 2009 Mar 11;21(10):104202. doi: 10.1088/0953-8984/21/10/104202. Epub 2009 Feb 10.
Over the last eight years, a large number of x-ray absorption near edge structure (XANES) and/or electron energy loss near edge structure (ELNES) spectroscopic calculations for complex oxides and nitrides have been performed using the supercell-OLCAO (orthogonalized linear combination of atomic orbitals) method, obtaining results in very good agreement with experiments. The method takes into account the core-hole effect and includes the dipole matrix elements calculated from ab initio wavefunctions. In this paper, we describe the method in considerable detail, emphasizing the special advantages of this method for large complex systems. Selected results are reviewed and several hitherto unpublished results are also presented. These include the Y K edge of Y ions segregated to the core of a Σ31 grain boundary in alumina, O K edges of water molecules, C K edges in different types of single walled carbon nanotubes, and the Co K edge in the cyanocobalamin (vitamin B(12)) molecule. On the basis of these results, it is argued that the interpretation of specific features of the calculated XANES/ELNES edges is not simple for complex material systems because of the delocalized nature of the conduction band states. The long-standing notion of the 'fingerprinting' technique for spectral interpretation of experimental data is not tenable. A better approach is to fully characterize the structure under study, using either crystalline data or accurate ab initio modeling. Comparison between calculated XANES/ELNES spectra and available measurements enables us to ascertain the validity of the modeled structure. For complex crystals or structures, it is necessary to use the weighted sum of the spectra from structurally nonequivalent sites for comparison with the measured data. Future application of the supercell-OLCAO method to complex biomolecular systems is also discussed.
在过去八年中，已经使用超胞-正交线性组合原子轨道（OLCAO）方法对复杂氧化物和氮化物进行了大量的X射线吸收近边结构（XANES）和/或电子能量损失近边结构（ELNES）光谱计算，得到的结果与实验非常吻合。该方法考虑了芯孔效应，并包括从第一性原理波函数计算得到的偶极矩阵元。在本文中，我们详细描述了该方法，强调了该方法对于大型复杂系统的特殊优势。回顾了一些选定的结果，并展示了一些迄今未发表的结果。这些结果包括在氧化铝中Σ31晶界核心处偏析的Y离子的Y K边、水分子的O K边、不同类型单壁碳纳米管中的C K边以及钴胺素（维生素B12）分子中的Co K边。基于这些结果，有人认为，由于导带态的离域性质，对于复杂材料系统，计算得到的XANES/ELNES边的特定特征的解释并不简单。长期以来用于实验数据光谱解释的“指纹识别”技术的概念是站不住脚的。更好的方法是使用晶体数据或精确的第一性原理建模来全面表征所研究的结构。计算得到的XANES/ELNES光谱与现有测量结果之间的比较使我们能够确定建模结构的有效性。对于复杂晶体或结构，有必要使用来自结构不等价位点的光谱的加权和与测量数据进行比较。还讨论了超胞-OLCAO方法在复杂生物分子系统中的未来应用。
