Electronic structure and magnetic properties of small manganese oxide clusters

To investigate the electronic structure and magnetic properties of manganese oxide clusters, we carried out first-principles electronic structure calculations for small MnO clusters. Among various structural and magnetic configurations of the clusters, the bulklike [111]-antiferromagnetic ordering is found to be favored energetically, while the surface atoms of the clusters exhibit interesting electronic and magnetic characteristics which are different from their bulk ones. The distinct features of the surface atoms are mainly attributed to the reduction of Mn coordination numbers and the bond-length contractions in the clusters, which may serve as a key factor for the understanding of physical and chemical properties of magnetic oxide nanoparticles.     

A comparison of SCF-Xα and extended Hückel methods for metal clusters

A comparison of some results from Xα-scattered wave (Xα-SW) and extended Hückel (EH) calculations for metal clusters is given. It is found that small clusters of atoms (≈ 13 atoms) using the Xα-SW method reproduce many of the features of the electronic structure of the bulk metals, whereas this is not the case for the same clusters using the EH method. A more systematic approach to EH parametrizations is suggested in order to make this method a more viable approach to treating metal clusters.     

A Density Functional Study of Small （NiTi）x Clusters with x=1-3

Systematic studies on the geometry, electronic structure and vibrational properties of small (NiTi)x clusters with x=1-3, within the framework of the density functional theory, were performed in this work. The electronic structure analyses were used to investigate the bonding between the early-late(EL) transition metals in their alloy. The results of our calculations have been used to predict (NiTi)x clusters structure for the ground state and confirm that s-electron density is transferred from Ti atoms to Ni atoms followed by the back donation of electrons through the d-orbital. The study provides a fragment approach based on the isolobal analogy that NiTi molecular fragments play a significant role in forming clusters.     

Size dependence of the structures and energetic and electronic properties of gold clusters

The structures and stabilities of gold clusters with up to 14 atoms have been determined by density-functional theory. The structure optimizations and frequency analysis are performed with the Perdew-Wang 1991 gradient-corrected functional combined with the effective core potential and corresponding valence basis set (LANL2DZ). The turnover point from two-dimensional to three-dimensional geometry for gold clusters occurs at Au12. The energetic and electronic properties of the small gold clusters are strongly dependent on sizes and structures, which are in good agreement with experiment and other theoretical calculations. The even-odd oscillation in cluster stability and electronic properties predicted that the clusters with even numbers of atoms were more stable than the neighboring clusters with odd numbers of atoms. The stability and electronic structure properties of gold clusters are also characterized by the maximum hardness principle of chemical reactivity and minimum polarizability principle.     

Reductive Activation of Small Molecules by Anionic Coinage Metal Atoms and Clusters in the Gas Phase

Metal atoms and clusters exhibit chemical properties that are significantly different or totally absent in comparison to their bulk counterparts. Such peculiarity makes them potential building units for the generation of novel catalysts. Investigations of the gas‐phase reactions between size‐ and charge‐selected atoms/clusters and small molecules have provided fundamental insights into their intrinsic reactivity, thus leading to a guiding principle for the rational design of the single‐atom and cluster‐based catalysts. Especially, recent gas‐phase studies have elucidated that small molecules such as O₂, CO₂, and CH₃I can be catalytically activated by negatively‐charged atoms/clusters via donation of a partial electronic charge. This Minireview showcases typical examples of such “reductive activation” processes promoted by anions of metal atoms and clusters. Here, we focus on anionic atoms/clusters of coinage metals (Cu, Ag, and Au) owing to the simplicity of their electronic structures. The determination of a correlation between their activation modes and the electronic structures might be helpful for the future development of innovative coinage metal catalysts.     

Geometrical and electronic properties of neutral and charged cesium clusters Cs(n) (n=2-10): a theoretical study

We have investigated the structure and electronic properties of cesium clusters following all electron ab initio theoretical methods based on configuration interaction, second-order Moller-Plesset (MP2) perturbation theory, and density-functional theory. Becke's three-parameter nonlocal hybrid exchange-correlation functional (B3LYP) is found to perform best on the present systems with a split valence 3-21G basis function. We have calculated the optimized geometries of neutral and singly charged cesium clusters having up to ten atoms, their binding energy per atom, ionization potentials (IPs), and adiabatic electron affinity (EA). Geometry optimizations for all the clusters are carried out without imposing any symmetry restriction. The neutral clusters having up to six atoms prefer planar structure and three-dimensional structure is preferred only when the number of atoms in a cluster is more than six. There is a good agreement between the present theoretical and reported experimental IP values for the neutral clusters with cluster size n相似文献   

ER-BOC calculations of crystal bulk properties from smallest-cluster models

Ab initio calculation of bulk properties of crystals with a high accuracy, which is a long-time goal of solid chemistry and physics, is still difficult and expensive because a large cluster is required as a crystal structure model. This article proposes a model based on density functional theory (DFT) quantum chemistry calculations and the assumption that the bond order of a given atom with its nearest atoms in a compound is conserved over the entire range from its diatomic molecules to clusters and further to crystals. This entire range bond order conservation (ER-BOC) provides an effective way to correlate bulk properties of crystals with those of the corresponding molecules and small clusters. By combining this ER-BOC principle with hybrid DFT quantum chemistry calculations, accurate predictions of the bulk bond lengths of a crystal can be made using calculations on small clusters.     

Evolution of the properties of Al(n)N(n) clusters with size

A global optimization of stoichiometric (AlN)(n) clusters (n = 1-25, 30, 35, ..., 95, 100) has been performed using the basin-hopping (BH) method and describing the interactions with simple and yet realistic interatomic potentials. The results for the smaller isomers agree with those of previous electronic structure calculations, thus validating the present scheme. The lowest-energy isomers found can be classified in three different categories according to their structural motifs: (i) small clusters (n = 2-5), with planar ring structures and 2-fold coordination, (ii) medium clusters (n = 6-40), where a competition between stacked rings and globular-like empty cages exists, and (iii) large clusters (n > 40), large enough to mix different elements of the previous stage. All the atoms in small and medium-sized clusters are in the surface, while large clusters start to display interior atoms. Large clusters display a competition between tetrahedral and octahedral-like features: the former lead to a lower energy interior in the cluster, while the latter allow for surface terminations with a lower energy. All of the properties studied present different regimes according to the above classification. It is of particular interest that the local properties of the interior atoms do converge to the bulk limit. The isomers with n = 6 and 12 are specially stable with respect to the gain or loss of AlN molecules.     

Observable Structures of Small Neutral and Anionic Gold Clusters

Since gold clusters have mostly been studied theoretically by using DFT calculations, more accurate studies are of importance. Thus, small neutral and anionic gold clusters (Au_n and Au_n^?, n=4–7) were investigated by means of coupled cluster with singles, doubles, and perturbative triple excitations [CCSD(T)] calculations with large basis sets, and some differences between DFT and CCSD(T) results are discussed. Interesting isomeric structures that have dangling atoms were obtained. Structures having dangling atoms appear to be stable up to n=4 for neutral gold clusters and up to n=7 for anionic clusters. The relative stabilities and electronic properties of some isomers and major structures are discussed on the basis of the CCSD(T) calculations. This accurate structure prediction of small gold clusters corresponding to experimental photoelectron spectral peaks is valuable in the field of atom‐scale materials science including nanocatalysts.     

Semiempirical and ab-initio calculations of semiconductor clusters: variation of structure and symmetry with size for different compounds

Semiempirical (by extended Hückel method) and ab initio RHF SCF calculations are used for the wide range of cluster structures M_xX_y, where M = Cd,Ag; X = S,I: semiempirical - up to M₂₀X₃₅, and ab initio - for small clusters less than ten atoms. Variation of electronic structure with size for the fragments with tetrahedral coordination (bulklike sphalerite structures) and for some clusters of the lower symmetry allows to predict their possible geometries which are compared with experimental data. The chemical bonding factor (the chemical nature of bounded atoms, coordination number for metal and non-metal atoms, hybridization, etc) is of more importance in properties of the clusters than the familiar quantum confinement effect of semiconductor clusters (like CdS, CdSe, PbS, etc. ). The essential difference in regularities of small cluster formation is analysed for CdS- and AgI- based structures.     

Structure and electronic properties of PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters: theoretical investigations based on first principles calculations

A systematic theoretical study of the PbnM (M=C, Al, In, Mg, Sr, Ba, and Pb; n=8, 10, 12, and 14) clusters have been investigated to explore the effect of impurity atoms on the structure and electronic properties of lead clusters. The calculations were carried out using the density functional theory with generalized gradient approximation for exchange-correlation potential. Extensive search based on large numbers of initial configurations has been carried out to locate the stable isomers of PbnM clusters. The results revealed that the location of the impurity atom depends on the nature of interaction between the impurity atom and the host cluster and the size of the impurity atom. Whereas, the impurity atoms smaller than Pb favor to occupy the endohedral position, the larger atoms form exohedral capping of the host cluster. The stability of these clusters has been analyzed based on the average binding energy, interaction energy of the impurity atoms, and the energy gap between the highest occupied and lowest unoccupied energy levels (HLG). Based on the energetics, it is found that p-p interaction dominates over the s-p interaction and smaller size atoms interact more strongly. The stability analysis of these clusters suggests that, while the substitution of Pb by C or Al enhances the stability of the Pbn clusters, Mg lowers the stability. Further investigations of the stability of PbnM clusters reveal that the interplay between the atomic and electronic structure is crucial to understand the stability of these clusters. The energy gap analysis reveals that, while the substitution of Mg atom widens the HLG, all other elements reduce the gap of the PbnM clusters.     

The structures and electronic states of zinc-water clusters Zn(n)(H2O)(m) (n = 1-32 and m = 1-3)

Ab initio and Density Functional Theory (DFT) calculations have been carried out for zinc-water clusters Zn(n)-(H2O)(m) (n = 1-32 and m = 1-3, where n and m are the numbers of zinc atoms and water molecules, respectively) to elucidate the structure and electronic states of the clusters and the interaction of zinc cluster with water molecules. The binding energies of H2O to zinc clusters were small at n = 2-3 (2.3-4.2 kcal mol(-1)), whereas the energy increased significantly in n = 4 (9.0 kcal mol(-1)). Also, the binding nature of H2O was changed at n = 4. The cluster size dependency of the binding energy of H2O accorded well with that of the natural population of electrons in the 4p orbital of the zinc atom. In the larger clusters (n > 20), it was found that the zinc atoms in surface regions of the zinc cluster have a positive charge, whereas those in the interior region have a negative charge with the large electron population in the 4p orbital. The interaction of H2O with the zinc clusters were discussed on the basis of the theoretical results.     

Geometrical and electronic structure of small copper nanoclusters: XANES and DFT analysis

The geometrical and electronic structure of small copper nanoclusters was studied by density functional theory (DFT) and analysis of X-ray absorption spectra. It was shown that the icosahedral geometry of small copper nanoclusters of 13 atoms was energetically more stable than cuboctahedral geometry. The binding energies in these structures were compared; the theoretical XANES spectra were compared with experiment. The paper gives the results of ab initio calculations of the electronic structure of copper clusters differing in size.     

金钯二元小团簇的几何结构与电子性质

在UBP86/LANL2DZ和UB3LYP/def2-TZVP水平下详细研究了AumPdn(m+n≤6)团簇的几何结构和电子性质.阐明了团簇的结构特征、平均结合能、垂直电离势、垂直电子亲和能、电荷转移以及成键特征.除单取代混合团簇(AunPd和AuPdn,n=5或6)外,五和六原子混合团簇中钯原子趋于聚集到一起形成Pdcore,金原子分布在Pdcore周围形成PdcoreAushell结构.含一个和两个钯原子团簇的电子性质与纯金团簇类似,呈现一定奇偶振荡.混合团簇的电子性质,如最高占据分子轨道(HOMO),最低未占据分子轨道(LUMO),垂直电离势,垂直电子亲和能,Fermi能级和化学硬度等均与团簇空间结构和金、钯原子数之比直接相关.混合团簇中存在钯原子到金原子间的电荷转移,表明团簇中存在明显金钯间成键作用.分析团簇的电荷分布、前线轨道和化学硬度表明,金钯混合团簇对小分子如O2、H2和CO等的反应活性要强于纯金团簇.     

The H and H2 interaction with Pd3Cu,Pd4, and Cu4 fcc (111) clusters: A DFT comparative study

A comparative study of adsorption of H atoms and H₂ molecules on Pd₃Cu, Cu₄, and Pd₄ clusters has been performed through density functional calculations, using the hybrid B3LYP exchange‐correlation functional as implemented in the Gaussian98 program. For Pd atoms the relativistic small‐core effective core potential LANL and LANL2DZ basis set was used and for hydrogen a 6‐31G** basis set was used. The main emphasis is set in the reaction behavior of the different clusters with hydrogen atoms and molecules. We find that full geometry optimization does not appreciably change the metal cluster geometry either for certain reaction modes or the H and H₂ capture parameters, but increases the number of reactive sites of the metal clusters. Also, we found that there is charge transfer competition between H and Cu atoms, which drastically diminishes H₂ adsorption energy, related to the Pd cluster observed value. Edges and threefold sites are the principal hydrogen adsorption sites. Hydrogen has a great mobility over the metal clusters for different minima, especially when Cu is present; many initial pathways end in the same adsorption site. The observed hydrogen adsorption and binding energies are well reproduced by the calculations. Also, the adsorption mechanisms were determined. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Breakdown of the pseudopotential approximation for magnetizabilities and electric multipole moments: test calculations for Au, AuF, and Sn(n) cluster (n ≤ 20)

The response of the electronic wavefunction to an external electric or magnetic field is widely considered to be a typical valence property and should, therefore, be adequately described by accurately adjusted pseudopotentials, especially if a small-core definition is used within this approximation. In this paper we show for atomic Au and Au(+), as well as for the molecule AuF and tin clusters, that in contrast to the case of the static electric dipole polarizability or the electric dipole moment, core contributions to the static magnetizability are non-negligible, and can therefore lead to erroneous results within the pseudopotential approximation. This error increases with increasing size of the core chosen. For tin clusters, which are of interest in ongoing molecular beam experiments currently carried out by the Darmstadt group, the diamagnetic and paramagnetic isotropic components of the magnetizability tensor almost cancel out and large-core pseudopotentials do not even predict the correct sign for this property due to erroneous results in both the diamagnetic and (more importantly) the paramagnetic terms. Hence, all-electron calculations or pseudopotentials with very small cores are required to adequately predict magnetizabilities for atoms, molecules and the solid state, making it computationally more difficult to obtain this quantity for future investigations in heavy atom containing molecules or clusters. We also demonstrate for this property that all-electron density functional calculations are quite robust and give results close to wavefunction based methods for the atoms and molecules studied here.     

Emerging polymorphism in nanostructured TiO2: Quantum chemical comparison of anatase,rutile, and brookite clusters

Density functional theory (DFT) and time‐dependent DFT calculations have been performed on a set of 34 titanium dioxide clusters ((TiO2)_n with n ≤ 125) to investigate structural and electronic properties of nanostructured TiO₂ (nano‐TiO₂) materials. The investigated clusters include models of the three low‐energy polymorphic forms of TiO₂ anatase, rutile, and brookite. A systematic comparison of clusters of increasing size show clear trends for emerging bulk properties in the investigated systems as the surface‐to‐bulk ratio changes from small clusters dominated by undercoordinated surface atoms to more realistic model nanocrystals with significant bulk components. Differences and similarities in terms of atomic coordination, structural stability, and electronic properties for the three different polymorphic forms of nano‐TiO₂ are discussed. The calculations provide evidence for emerging polymorphism with increasing cluster sizes so that the different TiO₂ forms can be clearly distinguished based on structural characteristics associated with the local bonding environment of the constituent atoms. © 2013 Wiley Periodicals, Inc.     

Theoretical study of hydrogenated Mg, Ca@Al12 clusters

The studies on the structure and electronic properties of hydrogenated metal embedded Al(12) cage clusters have been performed by density functional theory calculations. We have investigated aluminum cluster hydrides with 12 and 14 hydrogen atoms, respectively. Insertion of the Mg, Ca alkali metals remarkably enhances the stability of the aluminum clusters. The hydrogen atom prefers to occupy on-top sites along the surface of the clusters. Mulliken population analysis indicates that significant charge transfer occurs between the Mg and Ca atoms and the Al atoms. Our computations suggest that these clusters appear to be physically and chemically stable.