Yoo Soohaeng, Zeng X C
Department of Chemistry and Center for Materials Research and Analysis, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA.
J Chem Phys. 2005 Oct 22;123(16):164303. doi: 10.1063/1.2043127.
It has been established from experiments that stable medium-sized ionic clusters Si15-Si20 are prolate in shape. Density-functional theories (DFTs) also predict that nearly all low-lying neutral clusters in this size range are prolate in shape. Moreover, most of them are built onto two generic structural motifs, either the tricapped-trigonal-prism (TTP) Si9 motif or the six/six Si6Si6 (sixfold-puckered hexagonal ring Si6 plus six-atom tetragonal bipyramid Si6) motif. However, it appears that the exact location of the TTP-to-six/six motif transition is dependent on the functional (e.g., PBE or BLYP) used in the DFT calculations. Here, we present total-energy calculations for two series of clusters (one series containing six/six motif and the other containing the TTP motif) in the size range of Si16-Si20. The calculations were based on all-electron DFT methods with a medium [6-311G (2d)] and a large (cc-pVTZ) basis sets, as well as coupled-cluster single and double substitutions (including triple excitations) [CCSD(T)] method with a modest (cc-pVDZ) basis set. In the DFT calculations, two popular hybrid density functionals, the B3LYP and PBE1PBE, were selected. It is found that the B3LYP total-energy calculations slightly favor the six/six motif, whereas the PBE1PBE calculations slightly favor the TTP motif. The CCSD(T) total-energy calculations, however, show that isomers based on the six/six motif are energetically slightly favorable in the size range of Si16-Si20. Hence, the TTP-to-six/six motif transition is more likely to occur at Si16.
实验已经证实，稳定的中等尺寸离子团簇Si15 - Si20呈长形。密度泛函理论（DFT）也预测，这个尺寸范围内几乎所有低能中性团簇都呈长形。此外，它们中的大多数基于两种通用结构模式构建，即三帽三棱柱（TTP）Si9模式或六/六Si6Si6（六重褶皱六边形环Si6加上六原子四方双锥Si6）模式。然而，TTP到六/六模式转变的确切位置似乎取决于DFT计算中使用的泛函（例如PBE或BLYP）。在这里，我们给出了Si16 - Si20尺寸范围内两个系列团簇（一个系列包含六/六模式，另一个包含TTP模式）的总能计算结果。计算基于全电子DFT方法，使用中等[6 - 311G(2d)]和大（cc - pVTZ）基组，以及耦合簇单双取代（包括三重激发）[CCSD(T)]方法和适度（cc - pVDZ）基组。在DFT计算中，选择了两种流行的杂化密度泛函B3LYP和PBE1PBE。结果发现，B3LYP总能计算略微倾向于六/六模式，而PBE1PBE计算略微倾向于TTP模式。然而，CCSD(T)总能计算表明，在Si16 - Si20尺寸范围内，基于六/六模式的异构体在能量上略微有利。因此，TTP到六/六模式的转变更可能发生在Si16处。
