Conductance through atomic point contacts between fcc(100) electrodes of gold

Electrical conductance through various nanocontacts between gold electrodes is studied by using the density functional theory, scalar-relativistic pseudopotentials, generalized gradient approximation for the exchange-correlation energy and the recursion-transfer-matrix method along with channel decomposition. The nanocontact is modeled with pyramidal fcc(100) tips and 1 to 5 gold atoms between the tips. Upon elongation of the contact by adding gold atoms between the tips, the conductance at Fermi energy E_F evolves from G ~ 3 G_0 to G ~ 1 G_0 (G_0 = 2e/h^2). Formation of a true one-atom point contact, with G ~ 1 G_0 and only one open channel, requires at least one atom with coordination number 2 in the wire. Tips that share a common vertex atom or tips with touching vertex atoms have three partially open conductance channels at E_F, and the symmetries of the channels are governed by the wave functions of the tips. The long 5-atom contact develops conductance oscillations and conductance gaps in the studied energy range -3 < E-E_F < 5 eV, which reflects oscillations in the local density of electron states in the 5-atom linear "gold molecule" between the electrodes, and a weak coupling of this "molecule" to the tips

Quantum walks on Cayley graphs

We address the problem of the construction of quantum walks on Cayley graphs. Our main motivation is the relationship between quantum algorithms and quantum walks. In particular, we discuss the choice of the dimension of the local Hilbert space and consider various classes of graphs on which the structure of quantum walks may differ. We completely characterise quantum walks on free groups and present partial results on more general cases. Some examples are given, including a family of quantum walks on the hypercube involving a Clifford Algebra.Comment: J. Phys. A (accepted for publication

A QM/MM approach for the study of monolayer-protected gold clusters

We report the development and implementation of hybrid methods that combine quantum mechanics (QM) with molecular mechanics (MM) to theoretically characterize thiolated gold clusters. We use, as training systems, structures such as Au25(SCH2-R)18 and Au38(SCH2-R)24, which can be readily compared with recent crystallographic data. We envision that such an approach will lead to an accurate description of key structural and electronic signatures at a fraction of the cost of a full quantum chemical treatment. As an example, we demonstrate that calculations of the 1H and 13C NMR shielding constants with our proposed QM/MM model maintain the qualitative features of a full DFT calculation, with an order-of-magnitude increase in computational efficiency.Comment: Journal of Materials Science, 201

First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands

Bestowing chirality to metals is central in fields such as heterogeneous catalysis and modern optics. Although the bulk phase of metals is symmetric, their surfaces can become chiral through adsorption of molecules. Interestingly, even achiral molecules can lead to locally chiral, though globally racemic, surfaces. A similar situation can be obtained for metal particles or clusters. Here we report the first separation of the enantiomers of a gold cluster protected by achiral thiolates, Au38(SCH2CH2Ph)24, achieved by chiral high-performance liquid chromatography. The chirality of the nanocluster arises from the chiral arrangement of the thiolates on its surface, forming 'staple motifs'. The enantiomers show mirror-image circular dichroism responses and large anisotropy factors of up to 4×10−3. Comparison with reported circular dichroism spectra of other Au38 clusters reveals that the influence of the ligand on the chiroptical properties is minor

Current Applications of Computational Chemistry in JACS

Article discussing molecules, mechanisms, and materials and current applications of computational chemistry in the Journal of the American Chemical Society (JACS)

Electronic Structure and Bonding of Icosahedral Core-Shell Gold-Silver Nanoalloy Clusters Au_(144-x)Ag_x(SR)_60

Atomically precise thiolate-stabilized gold nanoclusters are currently of interest for many cross-disciplinary applications in chemistry, physics and molecular biology. Very recently, synthesis and electronic properties of "nanoalloy" clusters Au_(144-x)Ag_x(SR)_60 were reported. Here, density functional theory is used for electronic structure and bonding in Au_(144-x)Ag_x(SR)_60 based on a structural model of the icosahedral Au_144(SR)_60 that features a 114-atom metal core with 60 symmetry-equivalent surface sites, and a protecting layer of 30 RSAuSR units. In the optimal configuration the 60 surface sites of the core are occupied by silver in Au_84Ag_60(SR)_60. Silver enhances the electron shell structure around the Fermi level in the metal core, which predicts a structured absorption spectrum around the onset (about 0.8 eV) of electronic metal-to-metal transitions. The calculations also imply element-dependent absorption edges for Au(5d) \rightarrow Au(6sp) and Ag(4d) \rightarrow Ag(5sp) interband transitions in the "plasmonic" region, with their relative intensities controlled by the Ag/Au mixing ratio.Comment: 4 figure

Interaction between sugar and abscisic acid signalling during early seedling development in Arabidopsis

Sugars regulate important processes and affect the expression of many genes in plants. Characterization of Arabidopsis (Arabidopsis thaliana) mutants with altered sugar sensitivity revealed the function of abscisic acid (ABA) signalling in sugar responses. However, the exact interaction between sugar signalling and ABA is obscure. Therefore ABA deficient plants with constitutive ABI4 expression (aba2-1/35S::ABI4) were generated. Enhanced ABI4 expression did not rescue the glucose insensitive (gin) phenotype of aba2 seedlings indicating that other ABA regulated factors are essential as well. Interestingly, both glucose and ABA treatment of Arabidopsis seeds trigger a post-germination seedling developmental arrest. The glucose-arrested seedlings had a drought tolerant phenotype and showed glucose-induced expression of ABSCISIC ACID INSENSITIVE3 (ABI3), ABI5 and LATE EMBRYOGENESIS ABUNDANT (LEA) genes reminiscent of ABA signalling during early seedling development. ABI3 is a key regulator of the ABA-induced arrest and it is shown here that ABI3 functions in glucose signalling as well. Multiple abi3 alleles have a glucose insensitive (gin) phenotype comparable to that of other known gin mutants. Importantly, glucose-regulated gene expression is disturbed in the abi3 background. Moreover, abi3 was insensitive to sugars during germination and showed sugar insensitive (sis) and sucrose uncoupled (sun) phenotypes. Mutant analysis further identified the ABA response pathway genes ENHANCED RESPONSE TO ABA1 (ERA1) and ABI2 as intermediates in glucose signalling. Hence, three previously unidentified sugar signalling genes have been identified, showing that ABA and glucose signalling overlap to a larger extend than originally thought

Density functional theory based screening of ternary alkali-transition metal borohydrides: A computational material design project

The dissociation of molecules, even the most simple hydrogen molecule, cannot be described accurately within density functional theory because none of the currently available functionals accounts for strong on-site correlation. This problem led to a discussion of properties that the local Kohn-Sham potential has to satisfy in order to correctly describe strongly correlated systems. We derive an analytic expression for the nontrivial form of the Kohn-Sham potential in between the two fragments for the dissociation of a single bond. We show that the numerical calculations for a one-dimensional two-electron model system indeed approach and reach this limit. It is shown that the functional form of the potential is universal, i.e., independent of the details of the two fragments.We acknowledge funding by the Spanish MEC (Grant No. FIS2007-65702-C02-01), “Grupos Consolidados UPV/EHU del Gobierno Vasco” (Grant No. IT-319-07), and the European Community through e-I3 ETSF project (Grant Agreement No. 211956).Peer reviewe