Structural and dynamical properties of transition metal clusters

Results of molecular dynamics simulation studies of structural and dynamical properties of 12-, 13-, and 14-atom transition metal clusters are presented. The calculations are carried out using a Gupta-like potential expressed in reduced units. The transformation to absolute units involves two size-dependent parameters which effectively convert the potential into a size-dependent one. The minimum energy geometries of the clusters are obtained through the technique of simulated thermal quenching. A melting-like transition is observed as the energy of the clusters is increased. A novel element of the transition is that it may involve a premelting state.     

Structural and electronic properties of hydrated MgO nanotube clusters

Hydrated MgO nanotube clusters are constructed and studied by the density functional theory at the B3LYP/6-31G(d) level. A strong exothermicity chemisorption reactivity of MgO nanotube clusters with water, which releases 137.5–171.8 kJ/mol. The averaged charge of Mg ions is steady, and presents a stronger ionic bonding. Mg ions are more sensitive to the coordination number. For the reaction of water onto clusters, electronic properties of hydrated clusters have remarkable change compared with anhydrous clusters.     

Structural and electronic properties of transition metal thiophosphates

From the viewpoint of metal coordination we examine the structural characteristics of several new members of transition metal thiophosphates (i.e., MPS phases with M = V, Nb, Ta), in which various ligands such as S²⁻, S²⁻₂, and phosphorus-sulfur polyanions P_nS^x−_m (1 ≤ n ≤ 4; 3 ≤ m ≤ 13; 2 ≤ x ≤ 6) provide either an octahedral or a bicapped prismatic coordination of the metal. Tight-binding band electronic structure calculations show that the low-lying acceptor orbitals responsible for lithium intercalation of thiophosphates are their d-block bands. This prediction is confirmed by our electrochemical lithium intercalation study which reveals that the reduction sites of thiophosphates are their metal cations. Molecular orbital calculations are carried out on vanadium compounds with extremely short interligand S···S contacts. The occurrence of such short contact distances is not caused by covalent bonding in the S···S contacts but by the small size of vanadium cations which forces its surrounding sulfur ligands to squeeze one another.     

General properties of the electronic structure of alkali metal clusters and Ia-IIa mixed clusters

The results of the systematic ab-initio CI investigation of neutral and charged Li _n , Na _n , BeLi _k and MgNa _k clusters are summarized and analyzed. The general characteristic features of the electronic structure are pointed out:a) The participation of the atomic orbitals, which are empty in Ia and IIa metal atoms, allows for a higher valency of these atoms in clusters.b) Jahn-Teller and pseudo-Jahn-Teller effects strongly influence the electronic and geometric structure of clusters.c) Deformations of cluster geometry can lead to biradicaloid structures with higher spin multiplicity in their ground states.d) The peculiarities of the electronic structures of clusters can be deduced from the presence of many “surface” atoms. The theoretical results agree with experimental data presently available and they are useful for interpretation of the experimental findings.     

Structural, energetic, and electronic properties of hydrogenated titanium clusters

Hydrogen undergoes dissociative chemisorption on small titanium clusters. How the electronic structure of the cluster changes as a function of the number of adsorbed hydrogen atoms is an important issue in nanocatalysis and hydrogen storage. In this paper, a detailed theoretical investigation of the structural, energetic, and electronic properties of the icosahedral Ti13 cluster is presented as a function of the number of adsorbed hydrogen atoms. The results show that hydrogen loaded Ti13H20 and Ti13H30 clusters are exceptionally stable and are characterized by hydrogen multicenter bonds. In Ti13H20, the dissociated hydrogen atoms are bound to each of the 20 triangular faces of Ti13, while in Ti13H30, they are bound to the 30 Ti-Ti edges of Ti13. Consequently, the chemisorption and desorption energies of the Ti13H20 (1.93 eV, 3.10 eV) are higher than that of Ti13H30 (1.13 eV, 1.95 eV). While increased hydrogen adsorption leads to an elongation of the Ti-Ti bonds, there is a concomitant increase in the electrostatic interaction between the dissociated hydrogen atoms and the Ti13 cluster. This enhanced interaction results from the participation of the subsurface titanium atom at higher hydrogen concentrations. Illustrative results of hydrogen saturation on the larger icosahedral Ti55 cluster are also discussed. The importance of these results on hydrogen saturated titanium clusters in elucidating the mechanism of hydrogen adsorption and desorption in titanium doped complex metal hydrides is discussed.     

Ionic and electronic structure of metal clusters

Structure and electron dynamics of sodium clusters are investigated within the local-density approximation for the electrons. We compare results from detailed ionic structure with those from a structure averaged jellium model and find that the dominance of the electron cloud overlays most of the differences in the background. Ionic structure is indispensable, however, to compute the surface energy of clusters and to provide an unprejudiced picture of cluster fission. For all cases, we compute the resonance spectra associated with electron dynamics. In particular, the very strong deformations during fission deliver unusual resonance modes with a broad spectral fragmentation.     

Structural and electronic properties of small beryllium clusters: a theoretical study

Geometric structures and electronic properties of small beryllium clusters (Be(n), 2< or = n< or =9) are investigated within the gradient-corrected density functional theory. The computations are performed with the Becke exchange and Perdew-Wang correlation functionals. Both low and high multiplicity states are considered. A predominance of higher multiplicity states among the low-energy isomers of the larger clusters is found. An analysis of the variations in the structural and electronic properties with cluster size is presented, and the results are compared with those of earlier studies.     

Structural, electronic, and chemical properties of multiply iodized aluminum clusters

The electronic structure, stability, and reactivity of iodized aluminum clusters, which have been investigated via reactivity studies, are examined by first-principles gradient corrected density functional calculations. The observed behavior of Al13I(x)- and Al14I(x)- clusters is shown to indicate that for x < or = 8, they consist of compact Al13- and Al14++ cores, respectively, demonstrating that they behave as halogen- or alkaline earth-like superatoms. For x > 8, the Al cores assume a cagelike structure associated with the charging of the cores. The observed mass spectra of the reacted clusters reveal that Al13I(x)- species are more stable for even x while Al14I(x)- exhibit enhanced stability for odd x(x > or = 3). It is shown that these observations are linked to the formation and filling of "active sites," demonstrating a novel chemistry of superatoms.     

Geometry and electronic properties of alkali metal (rubidium) doped boron clusters

Alkali metal-doped boron clusters have captured much attention because of their novel electronic properties and structural evolution. In the study of RbB_n^0/− (n = 2–12) clusters, the minimum global search of the potential energy surface and structure optimization at the level of PBE1PBE by using the CALYPSO method and Gaussian package coupled with DFT calculation; the geometrical structures and electronic properties are systematically investigated. At n = 8, the ground-state structures are composed of an Rb atom above B atoms, forming a structurally stable pagoda cone. By stability analysis and charge transfer calculation, the RbB₈⁻ cluster shows more stability. It found that s-p hybridization between Rb atom and B atoms as well as s-p hybridization between B atoms is one of the reasons for the outstanding stability exhibited in the RbB₈^0/− clusters by using DOS and HOMO–LUMO orbital contour maps. The chemical bonding of the RbB₈^0/− groups was analyzed by using the AdNDP method, and B atoms with larger numbers readily form multi-center chemical bonds with the Rb atom. From the results of the bonding analysis, the interaction between the Rb atom and B atoms strengthens the stability of the RbB₈^0/− clusters. It is hoped that this work provides a direction for experimental manipulation.     

Structural and electronic properties of boron-doped lithium clusters: ab initio and DFT studies

The lowest-energy structures and electronic properties of the BLi(n) (n = 1-7) clusters are reported using the B3LYP, MP2, and CCSD(T) methods with the aug-cc-pVDZ basis set. Though the results at the B3LYP level agree well with those at the CCSD(T) level, the MP2 method is rather unsatisfactory. The first three-dimensional ground state in the BLi(n) clusters occurs for BLi(4), and the impurity B atom is seen to be trapped in a Li cage from the BLi(6) cluster onwards. The evolution of the binding energies, vertical ionization potentials, and polarizability with size of cluster shows the BLi(5) cluster to be most stable among the BLi(n) clusters. Besides, the BLi(5) cluster is also found to have the largest reaction enthalpy (49.8 kcal/mol) upon losing a Li atom, which is different from the previous prediction. The unique stability of the 8-valence electron BLi(5) can be understood from the cluster electronic shell model (CSM). However, in contradiction to the prediction of the CSM, the 2s level is filled prior to the 1d level in the BLi(n) clusters.     

Structural and electronic properties of defective 2D transition metal dichalcogenide heterostructures

We present a first-principles study on the structure-property relationships in MoS₂ and WS₂ monolayers and their vertically stacked hetero-bilayer, with and without Sulfur vacancies, in order to dissect the electronic features behind their photocatalytic water splitting capabilities. We also benchmark the accuracy of three different exchange-correlation density functionals for both minimum-energy geometries and electronic structure. The best compromise between computational cost and qualitative accuracy is achieved with the HSE06 density functional on top of Perdew–Burke–Ernzerhof minima, including dispersion with Grimme's D3 scheme. This computational approach predicts the presence of mid-gap states for defective monolayers, in accordance with the present literature. For the heterojunction, we find unexpected vacancy-position dependent electronic features: the location of the defects leads either to mid-gap trap states, detrimental for photocatalyst or to a modification of characteristic type II band alignment behavior, responsible for interlayer charge separation and low recombination rates.     

Structural,electronic and magnetic properties of small gold clusters with a copper impurity

A set of all-electron scalar relativistic calculations on Au _n Cu (n = 1–12) clusters has been performed using density functional theory with the generalized gradient approximation at PW91 level. The lowest energy geometries of Au _n Cu clusters may be considered as assemblies of triangular Au₃ moieties substituted with one Cu atom at the highest coordinated site. All these lowest energy geometries of the Au _n Cu clusters are slightly distorted but retain the planar structures of the Au _n+1 clusters due to the strong scalar relativistic effects. The Au–Cu bonds are stronger, and a few Au–Au bonds far from the Cu atom are weaker, than the corresponding Au–Au bonds in pure Au _n+1 clusters. After doping with a Cu atom, the thermodynamic stability and chemical reactivity are enhanced to some extent. The odd-numbered Au _n Cu clusters with even numbers of valence electrons are more stable than the neighboring even-numbered Au _n Cu clusters with odd numbers of valence electrons. Odd–even alternations of magnetic moments and electronic configurations for the Au _n Cu clusters can be observed clearly and may be understood in terms of the electron pairing effect.     

Thermal electronic properties of alkali clusters

We apply the finite-temperature Kohn-Sham method to alkali metal clusters, using the spherical jellium model and treating the valence electrons as a canonical system in the heat bath of the ions. We study the shell effects in the total free energyF(N) and the entropyS(N) for neutral clusters containingN atoms. Their strongest temperature dependence is due to the finite ground-state valueS ₀>0 of the electronic entropy for non-magic clusters. It leads to a decreasing amplitude and an increasing smear-out of the saw-tooth structure in the first difference Δ₁ F(N)=F(N?1)?F(N) with increasing temperatureT and cluster sizeN.     

Structural investigations of metal clusters by HREM

Silver clusters grown by the inert gas aggregation technique have been investigated by HREM. Undistorted cuboctahedra and icosahedra together with twinned particles have been observed. Three types of nucleation and growth mechanisms are proposed which can explain the observed particle structures. The untwinned large particles are created by the coalescence of liquid- like small clusters. The twins are produced by the coalescence of solid subunits.     

Chemisorption and the properties of metal clusters

In an earlier [C.W. Bauschlicher, D.H. Liskow, C.F. Bender and H.F. Schaefer, J. Chem. Phys. 62 (1975) 4815] we reported model studies of the chemisorption of atomic hydrogen on the (0001) surface of metallic beryllium. Four distinct sites for chemisorption were considered and the surface was modeled by clusters as large as Be₁₀. In the present paper, this work has been extended in two directions. First, two distint Be₁₃ clusters have been studied in an analogous manner. Second, the predicted chemisorption characteristics have been correlated with several properties of the isolated metal clusters. Metal cluster properties investigated include the ionization potential, cohesive energy, and singlet—triplet separation. The various sites for chemisorption are compared and electronic structure considerations discussed.     

Structural and electronic properties of heptachlor

In this work, the molecular geometry of heptachlor is investigated using ab initio HF, DFT, LDA, and GGA methods. The natural bond orbital (NBO) analysis is performed at the B3LYP/6-311++G(d,p) level of theory. The first order hyperpolarizability β_total, the mean polarizability Δα, the anisotropy of the polarizability Δα, and the dipole moment μ, are calculated by B3LYP/6-311++G(d,p) and HF/6- 311++G(d,p) methods. The first order hyperpolarizability (β_total) is calculated based on the finite field approach. UV spectral parameters along with HOMO, LUMO energies for heptachlor are determined in vacuum and the solvent phase using HF, DFT, and TD-DFT/B3LYP methods implemented with the 6-311++G(d,p) basis set. Atomic charges and electron density of heptachlor in vacuum and ethanol are calculated using DFT/B3LYP and TD-DFT/B3LYP methods and the 6-311++G(d,p) basis set. In addition, after the frontier molecular orbitals (FMOs), the molecular electrostatic potential (MEP), the electrostatic potential (ESP), the electron density (ED), and the solvent accessible surface of heptachlor are visualized as a results of the B3LYP/6-311++G(d,p) calculation. Densities of states (DOS), the external electric field (EF) effect on the HOMO-LUMO gap, and the dipole moment are investigated by LDA and GGA methods.     

Shell structure and response properties of metal clusters

Electronic shell structure, which was first recognized in sodium clusters, has been observed in alkali and noble metals, as well as in divalent and trivalent metals. Shell structure with modifications is expected to be broadly applicable to most metals. Features in the cluster abundance spectra and in the experimental dipole polarizabilities and ionization potentials correlate well with predictions of electronic level filling in spherical and spheroidal potential wells. The lack of precise quantitative agreement between experiment and theory for the response properties indicates necessary refinements in the self-consistent uniform background jellium model for clusters.     

Structural and energetic properties of Ni-Cu bimetallic clusters

The lowest-energy structures for all compositions of Ni n Cu m bimetallic clusters with N = n + m up to 20 atoms, N = 23, and N = 38 atoms have been determined using a genetic algorithm for unbiased structure optimization in combination with an embedded-atom method for the calculation of the total energy for a given structure. Comparing bimetallic clusters with homoatomic clusters of the same size, it is shown that the most stable structures for each cluster size are composed entirely of Ni atoms. Among the bimetallic clusters in the size range N = 2-20, the Ni N-1 Cu 1 clusters possess the highest stability. Further, it has been established that most of the bimetallic cluster structures have geometries similar to those of pure Ni clusters. The size N = 38 presents a special case, as the bimetallic clusters undergo a dramatic structural change with increasing atom fraction of Cu. Moreover, we have identified an icosahedron, a double, and a triple icosahedron with one, two, and three Ni atoms at the centers, respectively, as particularly stable structures. We show that in all global-minimum structures Ni atoms tend to occupy mainly high-coordination inner sites, and we confirm the segregation of Cu on the surface of Ni-Cu bimetallic clusters predicted in previous studies. Finally, it is observed that, in contrast to the bulk, the ground-state structures of the 15-, 16-, and 17-atom bimetallic clusters do not experience a smooth transition between the structures of the pure copper and the pure nickel clusters as a function of the relative number of the two types of atoms. For these sizes, the concentration effect on energy is more important than the geometric one.