Novel superconducting structures of BH2 under high pressure.

The crystal structures of boron hydrides in a pressure range of 50-400 GPa were studied using the genetic algorithm (GA) method combined with first-principles density functional theory calculations. BH4 and BH5 are predicted to be thermodynamically unstable. Two new BH2 structures with Cmcm and C2/c space group symmetries, respectively, were predicted, in which the B atoms tend to form two-dimensional sheets. The calculated band structures showed that in the pressure range of 50-150 GPa, the Cmcm-BH2 phase has very small gaps, while the C2/c-BH2 phase at 200-400 GPa is metallic. The superconductivity of the C2/c-BH2 structure was also investigated, and electron-phonon coupling calculations revealed that the estimated Tc values of C2/c-BH2 are about 28.18-37.31 K at 250 GPa.

Theoretical Prediction of Si2-Si33 Absorption Spectra.

The optical absorption spectra of Si2-Si33 clusters were systematically studied by a time-dependent density functional theory approach. The calculations revealed that the absorption spectrum becomes significantly broad with increasing cluster size, stretching from ultraviolet to the infrared region. The absorption spectra are closely related to the structural motifs. With increasing cluster size, the absorption intensity of cage structures gradually increases, but the absorption curves of the prolate and the Y-shaped structures are very sensitive to cluster size. If the transition energy reaches ∼12 eV, it is noted that all the clusters have remarkable absorption in deep ultraviolet region of 100-200 nm, and the maximum absorption intensity is ∼100 times that in the visible region. Further, the optical responses to doping in the Si clusters were studied.

Geometric structures of Ge(n) (n=34-39) clusters.

The structures of Ge(n) (n=34-39) clusters were searched by a genetic algorithm using a tight-binding interatomic potential. First-principles calculations based on density functional theory were performed to further identify the lowest-energy structures. The calculated results show that Ge(n) (n=34-39) clusters favor prolate or Y-shaped three-arm structures consisting of two or three small stable clusters (Ge(6), Ge(7), Ge(9), or Ge(10)) linked by a Ge(6) or Ge(9) bulk unit. The calculated results suggest the transition point from prolate to Y-shaped three-arm structures appears at Ge(35) or Ge(36).

Structures and stabilities of Pb(n) (n

We have performed global structural optimizations for neutral lead clusters Pb(n) (n = 2-20) by using a genetic algorithm (GA) coupled with a tight-binding (TB) potential. The low-energy structures identified from a GA/TB search were further optimized at the DFT-PBE level. The calculated results show that the Pb(n) (14 < n

Bulklike structures for medium-sized Al(n) (n=31-40) clusters.

Neutral aluminum clusters Al(n) (n=31-40) were studied using a genetic algorithm (GA)/tight-binding (TB) search combined with DFT-PBE calculations. It is found that the medium-sized aluminum clusters Al(31) to Al(40) exhibit a bulklike stacking pattern. Anion clusters were also studied.

Platelike structures of semiconductor clusters Ge(n) (n = 40-44).

The structures of Ge(n) (n=40-44) clusters were searched by genetic algorithm combined with a tight-binding method. First-principles calculations based on density functional theory were performed to further optimize the isomer structures. The calculated results show that Ge(n) (n=40-44) clusters favor platelike structures, consisted of four small magic clusters (Ge(9) or Ge(10)), and a Ge(4) core. The Ge(4) core along with the parts of the four linked small clusters forms a diamond segment. The cluster mobilities of the most stable structures are in good agreement with the experimental data.

Theoretical study on the structures and optical absorption of Si172 nanoclusters.

The structures and optical properties of silicon nanoclusters (Si NCs) have attracted continuous interest in the last few decades. However, it is a great challenge to determine the structures of Si NCs for accurate property calculation due to the complication and competition of various structural motifs. In this work, a Si172 NC with a size of about 1.8 nm was investigated using a genetic algorithm combined with tight-binding and DFT calculations. We found that a diamond crystalline core with 50 atoms (1.2 nm) was formed in the Si172 NC. It can be expected that at a size of about 172 atoms, a diamond crystalline structure can nucleate from the center of the Si NCs. The optical properties of the pure and hydrogenated Si172 NC structures also have been studied using the TDDFT method. Compared with the pure Si172 NC, the absorption peaks of the hydrogenated Si172 NC are obviously blue-shifted.

An ab initio calculation study of silicon and carbon binary clusters C7Si(n) (n = 1-7).

Binary C(7)Si(n) (n = 1-7) clusters are studied using density functional calculations at the level of B3LYP/6-311G(d). Lowest-energy structures have been determined theoretically and their properties such as binding energies, second differences in energy and highest-occupied and lowest-unoccupied molecular orbital gaps have been analyzed. It is found that the lowest-energy structures of the C(7)Si(n) (n = 1-7) clusters change from linear to planar when n ≥ 3, and in the planar structures C atoms prefer to form five- and six-membered rings surrounded by extra Si atoms in the form of the C(2)Si units.

Stabilities and fragmentation energies of Si(n) clusters (n = 2-33).

The structures of Si(n) (n = 2-33) were confirmed by genetic algorithm (GA)/tight binding (TB) search and ab initio calculations at the B3LYP/6- 311++G(2d) and PW91/6-311++G(2d) level, respectively. The fragmentation energies, binding energies, second differences in energy, and highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps in the size range 2≤n≤33 were calculated and analyzed systematically. We extended the cluster size involved in the fragmentation analyses up to Si(33), and studied the multi-step fragmentations of Si(n). The calculated result is similar to the fragmentation behavior of small silicon clusters studied previously, showing that Si(6), Si(7), and Si(10) have relatively larger stabilities and appear more frequently in the fragmentation products of large silicon clusters, which is in good agreement with the experimental observations.