Structural properties of transition-metal clusters via force-biased Monte Carlo and ab initio calculations: A comparative study (original) (raw)
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Structure of transition metal clusters: A force-biased Monte Carlo approach
Journal of Physics: Conference Series
We present a force-biased Monte Carlo (FMC) method for structural modeling of transition metal clusters of Fe, Ni, and Cu with 5 to 60 atoms. By employing the Finnis-Sinclair potential for Fe and the Sutton-Chen potential for Ni and Cu, the total energy of the clusters is minimized using a method that utilizes atomic forces in Monte Carlo simulations. The structural configurations of the clusters obtained from this biased Monte Carlo approach are analyzed and compared with the same from the Cambridge Cluster Database (CCD). The results show that the total-energy of the FMC clusters is very close to the corresponding value of the CCD clusters as listed in the Cambridge Cluster Database. A comparison of the FMC and CCD clusters is presented by computing the pair-correlation function, the bond-angle distribution, and the distribution of atomic-coordination numbers in the first-coordination shell, which provide information about the two-body and three-body correlation functions, the local atomic structure, and the bonding environment of the atoms in the clusters.
Physical Review B, 2009
To assess the accuracy of exchange-correlation approximations within density functional theory ͑DFT͒, diffusion quantum Monte Carlo ͑DMC͒ and DFT methods are used to calculate the energies of isomers of three covalently bonded carbon and boron clusters ͑C 20 , B 18 , and B 20 ͒, and three metallic aluminum and copper clusters ͑Al 13 , Al 55 , and Cu 13 ͒. We find that local and semilocal DFT methods predict the same energy ordering as DMC for the metallic clusters but not for the covalent clusters, implying that the DFT functionals are inadequate in such systems. In addition, we find that DFT fails to describe energy reductions arising from Jahn-Teller distortions.
Physical Review B, 2009
In this study, the 13-atom cluster structures of alkaline metals, alkaline-earth metals, boron group metals, carbon group metals, and 3d, 4d, and 5d transition metals in the periodic table are investigated by density functional theory with three kinds of exchange-correlation ͑XC͒ functionals: ͑i͒ local-density approximation ͑LDA͒; ͑ii͒ generalized gradient approximation ͑GGA͒ with Perdew-Wang 91; and ͑iii͒ generalized gradient approximation with Perdew-Burke-Ernzerhof. The dependence on pseudopotentials ͑PPs͒ with and without semicore electrons is also examined. The relative energies of five selected high-symmetry three-dimensional and four low-symmetry layer-type isomers for each element of interest are calculated and studied. Among the 44 metallic 13-atom clusters, our results show that the two GGA XC functionals have a great consistency; LDA and GGA results also reveal a great consistency, apart from the Cr, Mn, Fe, Co, Ni, and Rh 13-atom clusters, for which the results show a significant difference. Meanwhile, for most of the elements, the calculations with and without semicore PPs also produce consistent results, except for Cr, Mo, and V, which require a careful treatment of semicore states in the PPs.
Transition-metal 13-atom clusters assessed with solid and surface-biased functionals
The Journal of chemical …, 2011
First-principles density-functional theory studies have reported open structures based on the formation of double simple-cubic (DSC) arrangements for Ru 13 , Rh 13 , Os 13 , and Ir 13 , which can be considered an unexpected result as those elements crystallize in compact bulk structures such as the face-centered cubic and hexagonal close-packed lattices. In this work, we investigated with the projected augmented wave method the dependence of the lowest-energy structure on the local and semilocal exchange-correlation (xc) energy functionals employed in density-functional theory. We found that the local-density approximation (LDA) and generalized-gradient formulations with different treatment of the electronic inhomogeneities (PBE, PBEsol, and AM05) confirm the DSC configuration as the lowest-energy structure for the studied TM 13 clusters. A good agreement in the relative total energies are obtained even for structures with small energy differences, e.g., 0.10 eV. The employed xc functionals yield the same total magnetic moment for a given structure, i.e., the differences in the bond lengths do not affect the moments, which can be attributed to the atomic character of those clusters. Thus, at least for those systems, the differences among the LDA, PBE, PBEsol, and AM05 functionals are not large enough to yield qualitatively different results.
BFW: A Density Functional for Transition Metal Clusters
The Journal of Physical Chemistry A, 2007
Ionization potentials (IPs) or electron affinities (EAs) for transition metal clusters are an important property that can be used to identify and differentiate between clusters. Accurate calculation of these values is therefore vital. Previous attempts using a variety of DFT models have correctly predicted trends, but have relied on the use of scaling factors to compare to experimental IPs. In this paper, we introduce a new density functional (BFW) that is explicitly designed to yield accurate, absolute IPs for transition metal clusters. This paper presents the numerical results for a selection of transition metal clusters and their carbides, nitrides, and oxides for which experimental IPs are known. When tested on transition metal clusters, the BFW functional is found to be significantly more accurate than B3LYP and B3PW91.
Local densities of states and bonding properties of 3d-transition metal clusters
Surface Science, 1976
Electronic structures have been calculated for 13-atom clusters of 3d-transition metal atoms. The cluster geometry has features of both stepped and flat surfaces and the results demonstrate certain trends which parallel those of the bulk: (1) broader d-bands and more extended d-orbitals for lighter elements, (2) states near the top of the band are anti-bonding and more contracted than the bonding states near the bottom of the band, (3) gross details of the densities of states of clusters and bulk are similar. Cluster energy distributions are, however, more complex due to the lower symmetry at the surface, and edge and corner atoms show pronounced charge redistributions as a result of interactions among neighbouring d-orbitals. Some consequences of the results are discussed, and an assessment made of the scattered-wave cluster technique for chemisorption studies.
Physical Review B, 1997
We calculate the electronic structure of 3d transition-metal clusters with a model Hamiltonian that takes into account electron spillover at the cluster surface and uses bulk parameter values for the interactions. We perform calculations for fcc and bcc clusters of up to 177 atoms making use of symmetry properties. We obtain magnetic moments and ionization potentials for Ni, Co, and Fe clusters, and compare with different experimental results inferring that the essential features of the electronic and magnetic properties are reproduced, starting from an spd-bulk parametrization, if a realistic approach for electron spillover is considered. ͓S0163-1829͑97͒05419-2͔
Massively Parallel Density Functional Theory Calculations of Large Transition Metal Clusters
2006
We report on ab initio density functional theory (DFT) calculations of structural properties of large elementary transition metal clusters with up to 561 atoms, corresponding to a diameter of about 2.5 nm, which is a relevant size for practical applications. The calculations were carried out on an IBM Blue Gene/L supercomputer, showing that reasonable scaling up to 1024 processors and beyond can be achieved with modern pseudopotential plane wave codes.
Theoretical Studies of Structural, Energetic, and Electronic Properties of Clusters
Zeitschrift für Physikalische Chemie, 2008
Size in combination with low symmetry makes theoretical studies of the properties of clusters a challenge. This is in particular the case when the studies also shall identify the structures of the lowest total energy. We discuss here various methods for calculating the structural, energetic, and electronic properties of nanoparticles, emphasizing that the computational method always should be chosen carefully according to the scientific questions that shall be addressed. Therefore, different approximate methods for calculating the total energy of a given structure are discussed, including the embedded-atom method and a parameterized density-functional method. Moreover, different approaches for choosing/determining the structures are presented, including an Aufbau/Abbau method and genetic algorithms. In order to illustrate the approaches we present results from calculations on metallic and semiconducting nanoparticles as well as on nanostructured HAlO.
Structure and energetics of nickel, copper, and gold clusters
The European Physical Journal D, 2005
The most stable structures of CuN , NiN , and AuN clusters with 2 ≤ N ≤ 60 have been determined using a combination of the embedded-atom (EAM), the quasi-Newton, and our own Aufbau/Abbau methods for the calculation of the total energy for a given structure, the structures of the local total-energy minima, and the structure of the global total-energy minimum, respectively. We have employed two well-known versions of the EAM: (1) the 'bulk' version of Daw, Baskes, and Foiles and the Voter-Chen version which takes into account also properties of the dimer in the parameterization. The lower-energy structures (also for the smallest) of CuN and NiN clusters (i.e., structural details as well as symmetry) obtained with the two versions are very similar. Thus, our study supports an universality of the bulk embedding functions for copper and nickel. But for gold clusters the differences between structures calculated with the two different versions of the EAM are significant, even for larger clusters.