Ab initio Molecular Dynamical Investigation of the Finite Temperature Behavior of the Tetrahedral Au19_{19} and Au20_{20} Clusters

Density functional molecular dynamics simulations have been carried out to understand the finite temperature behavior of Au$_{19}$ and Au$_{20}$ clusters. Au$_{20}$ has been reported to be a unique molecule having tetrahedral geometry, a large HOMO-LUMO energy gap and an atomic packing similar to that of the bulk gold (J. Li et al., Science, {\bf 299} 864, 2003). Our results show that the geometry of Au$_{19}$ is exactly identical to that of Au$_{20}$ with one missing corner atom (called as vacancy). Surprisingly, our calculated heat capacities for this nearly identical pair of gold cluster exhibit dramatic differences. Au$_{20}$ undergoes a clear and distinct solid like to liquid like transition with a sharp peak in the heat capacity curve around 770 K. On the other hand, Au$_{19}$ has a broad and flat heat capacity curve with continuous melting transition. This continuous melting transition turns out to be a consequence of a process involving series of atomic rearrangements along the surface to fill in the missing corner atom. This results in a restricted diffusive motion of atoms along the surface of Au$_{19}$ between 650 K to 900 K during which the shape of the ground state geometry is retained. In contrast, the tetrahedral structure of Au$_{20}$ is destroyed around 800 K, and the cluster is clearly in a liquid like state above 1000 K. Thus, this work clearly demonstrates that (i) the gold clusters exhibit size sensitive variations in the heat capacity curves and (ii) the broad and continuous melting transition in a cluster, a feature which has so far been attributed to the disorder or absence of symmetry in the system, can also be a consequence of a defect (absence of a cap atom) in the structure.Comment: 7 figure

Stability of the tetrahedral motif for small gold clusters in the size range 16-24 atoms

A 20-atom tetrahedral cluster of gold atoms is known to be anomalously stable, relative to more disordered structures. Here, we systematically investigate the nature of this stability and the extent to which it also applies to clusters derived from the tetrahedron by adding or removing Au atoms. © 2007 Elsevier B.V. All rights reserved

The electronic structure of Be and BeO: Benchmark EMS measurements and LCAO calculations

The electronic band structures of Be and BeO have been measured by transmission electron momentum spectroscopy (EMS). The low atomic number of beryllium and the use of ultrathin solid films in these experiments reduce the probability of electron multiple scattering within the sample, resulting in very clean 'benchmark' measurements for the EMS technique. Experimental data are compared to tight-binding (LCAO) electronic structure calculations using Hartree-Fock, and local density (LDA-VWN), gradient corrected (PBE) and hybrid (PBE0) density functional theory. Overall, DFT calculations reproduce the EMS data for metallic Be reasonably well. PBE predictions for the valence bandwidth of Be are in excellent agreement with EMS data, provided the calculations employ a large basis set augmented with diffuse functions. For BeO, PBE calculations using a moderately sized basis set are in reasonable agreement with experiment, slightly underestimating the valence bandgap and overestimating the O(2s) and O(2p) bandwidths. The calculations also underestimate the EMS intensity of the O(2p) band around the Γ-point. Simulation of the effects of multiple scattering in the calculated oxide bandstructures do not explain these systematic differences. Crown Copyright © 2002 Published by Elsevier Science Ltd. All rights reserved

Doelmatigheidswinst van minder hypotheekrenteaftrek

We bespreken enkele welvaartsdividenden van het versoberen van de subsidie op de eigen woning. Deze dividenden doen zich in de eerste plaats voor op de woningmarkt. Daarnaast kan zich een welvaartsdividend voordoen op de arbeidsmarkt. De omvang van dit tweede dividend is evenwel sterk afhankelijk van de elasticiteit van het aanbod van woningen

Saaie winkelstraten

Het lijkt alsof je overal dezelfde winkels ziet. Een eenvoudige analyse van de locatie van 2.156 winkels van negen ketens in 76 regio’s laat zien dat dit inderdaad zo is. De typische Nederlandse winkelstraat is overal ongeveer hetzelfde

A Short Evaluation of a New Haematological Analyser: The Cobas Argos 5 Diff

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