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首页> 外文期刊>Theoretical chemistry accounts >The subtlety of resolving orbital angular momenta in calculating Hubbard U parameters in the density functional tight-binding theory and its delicacy is illustrated by the calculated magnetic properties of carbon clusters
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The subtlety of resolving orbital angular momenta in calculating Hubbard U parameters in the density functional tight-binding theory and its delicacy is illustrated by the calculated magnetic properties of carbon clusters

机译：通过计算碳簇的磁特性说明了在密度函数紧绑定理论中计算轨道角度动量的分辨轨道角度动量的微妙性，碳簇的磁性特性说明了

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We study magnetic properties of carbon clusters C-n by combining the spin-polarized parametrized density functional tight-binding (SDFTB) theory with an unbiased modified basin hopping (MBH) optimization algorithm. With an intent to develop a physically self-consistent technique, we deliberately treat valence electronic charges, spin charges and ionic charges on equal footing. Within the density functional tight-binding (DFTB) theory, we examine the effect of using the orbital angular-momentum unresolved and resolved schemes in calculating the on-site Coulombic energy which, we judge, will have subtle but significant influence on the C-n's magnetism, their topologies and the change with size n their conformational structures. As a concrete means to substantiate our conjecture, we apply the SDFTB/MBH method to C(n)s and determine their stable magnetic structures within the angular-momentum resolved scheme. Our calculations show that the lowest energy C-n changes from a linear-chain shape for n = 3-9, turns over to a singlet monocyclic ring for n = 10-18, becomes a polycyclic ring for n = 19-25 and finally assumes a cage-like geometry at n = 26. Except for n = 4, 6 and 8, all other C(n)s are found unmagnetized. Accordingly, the newly discovered n that marks the first occurrence of a bi- to tridimensional transition occurs at n = 26, and this size is in contrast to n = 24 which was predicted by similar calculations using the unresolved scheme. Our calculations reveal furthermore two different features. The first one is that the predicted optimized geometries for all of the C(n)s, except for C-24, are structurally the same as the size n = 3-23 of C-n calculated by the DFTB/MBH employing the unresolved orbital angular-momentum scheme. As a result, the present calculations which employ the resolved angular-momentum scheme thus showed that the latter affects only the larger size C-n starting at C-24; this finding redefines therefore the turnover transition point of C-n from a bidimensional planar at n = 25 to a tridimensional cage-like at n = 26. The second feature is that the SDFTB/MBH method yields only triplet C-4, C-6 and C-8, whereas in our previous work employing DFTB/MBH, not only C-4 and C-6, all of C-13, C-15, C-17, C-19, C-22 and C-23 were predicted to carry a magnetic moment of 2 mu(B). These differences in the magnetism obtained are attributed to the combined use of both the SDFTB/MBH procedure and the orbital angular-momentum resolved scheme.
机译：通过将旋转偏振参数化函数紧密（SDFTB）理论与一个无偏的修改盆地跳跃（MBH）优化算法相结合，研究碳簇C-N的磁性。随着开发物理自我一致的技术的意图，我们刻意在等脚上进行旋转电荷，旋转电荷和离子电荷。在密度函数紧密（DFTB）理论中，我们研究了使用轨道角动势未解决和解决方案的效果在计算现场的库仑能量方面，我们判断，对CN'对CN'具有微妙但重要的影响S磁性，拓扑和尺寸与其构象结构的变化。作为证实我们的猜想的具体装置，我们将SDFTB / MBH方法应用于C（n）S并确定其在角动量分辨方案内的稳定磁结构。我们的计算表明，对于n = 3-9的线性链形，最低能量Cn从线性链形状变为N = 10-18的单环环，变为n = 19-25的多环环，最后假设a在n = 26处的笼状几何形状。除n = 4,6和8外，发现所有其他c（n）s未磁化。因此，新发现的n标记在n = 26处发生的第一转向转变的第一次出现，并且这种尺寸与n = 24相反，该尺寸是通过使用未解决方案的类似计算预测的n = 24。我们的计算揭示了两种不同的特征。第一个是除C-24之外的所有C（n）S的预测优化几何形状是由采用未解决的轨道角度计算的CN计算的CN的尺寸n = 3-23的结构相同-momentum方案。结果，采用分辨的角动量方案的本计算结果表明，后者仅在C-24开始的较大尺寸C-N;因此，该发现将CN的转换转换点从N = 25处的趋向平面的转换转换点重新定义为N = 26的三维笼状。第二特征是SDFTB / MBH方法产生三重态C-4，C-6和C-8，而在我们以前的工作中使用DFTB / MBH，而不仅是C-4和C-6，所有C-13，C-15，C-17，C-19，C-22和C-23都是预计携带2μ（b）的磁矩。所获得的磁性的这些差异归因于SDFTB / MBH程序和轨道角动量分辨方案的组合使用。

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