T Futschek

Bio: T Futschek is an academic researcher. The author has contributed to research in topics: Ab initio quantum chemistry methods & Ab initio. The author has an hindex of 2, co-authored 2 publications receiving 183 citations.

Structural and magnetic isomers of small Pd and Rh clusters: an ab initio density functional study

Abstract: We present a comprehensive investigation of the structural, electronic, and magnetic properties of PdN and RhN clusters with up to N = 13 atoms, based on ab initio density functional calculations. The novel aspects of our investigation are the following. (i) The structural optimization of the cluster by a symmetry-unconstrained static total-energy minimization has been supplemented for larger clusters (N≥7) with a search for the ground state structure by dynamical simulated annealing. The dynamical structural optimization has led to the discovery of highly unexpected ground state configurations. (ii) The spin-polarized calculations were performed in a fixed-moment mode. This allowed us to study coexisting magnetic isomers and led to a deeper insight into the importance of magnetostructural effects. For both Pd and Rh the larger clusters adopt ground state structures that can be considered as fragments of the face-centred cubic crystal structure of the bulk phase. For Pd clusters, the magnetic ground state is a spin triplet (S = 1) for N≤9, a spin quintuplet (S = 2) for N = 10, and a spin septet (S = 3) for the largest clusters. Large magnetic moments with up to S = 8 have been found for Rh clusters. Magnetic energy differences have been found to be small, such that there is an appreciable probability of finding excited magnetic isomers even at ambient temperatures. In several cases, the structural energy differences are also sufficiently small to permit the coexistence of polytypes.

Stable structural and magnetic isomers of small transition-metal clusters from the Ni group: an ab initio density-functional study

Abstract: We present ab initio density-functional studies of structural and magnetic isomers of NiN and PtN clusters with up to 13 atoms. Our investigations are based on fixed-moment calculations within a spin-polarized generalized gradient approximation and on static as well as dynamical optimizations of the cluster-structure, using quantum-mechanical many-body forces calculated via the Hellmann?Feynman theorem. Together with our earlier paper on PdN clusters (Futschek et al 2005 J.?Phys.:?Condens.?Matter 17 5927?63) the present work completes a comprehensive investigation of small clusters formed by metals of the Pt group of the periodic table. We discuss the trends in structure, binding energy and magnetic moments as a function of cluster size and through the 3d?4d?5d series. We demonstrate that the transition from the more localized 3d to the more extended 5d orbitals influences not only the magnetic ground state, but also the geometric structure of the clusters. The difference is most pronounced for the largest clusters in this series (N = 11,12,13) where the Ni clusters adopt a polytetrahedral arrangement converging to the Ni13 icosahedron, whereas the structures of Pd clusters and Pt clusters are based on octahedral motifs closely resembling fragments of the face-centred cubic structure of the bulk metals.

Ab-initio simulations of materials using VASP: Density-functional theory and beyond.

Abstract: During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science—promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008

Density functional theory investigation of 3d, 4d, and 5d 13-atom metal clusters

Abstract: The knowledge of the atomic structure of clusters composed by few atoms is a basic prerequisite to obtain insights into the mechanisms that determine their chemical and physical properties as a function of diameter, shape, surface termination, as well as to understand the mechanism of bulk formation. Due to the wide use of metal systems in our modern life, the accurate determination of the properties of $3d$, $4d$, and $5d$ metal clusters poses a huge problem for nanoscience. In this work, we report a density functional theory study of the atomic structure, binding energies, effective coordination numbers, average bond lengths, and magnetic properties of the $3d$, $4d$, and $5d$ metal (30 elements) clusters containing 13 atoms, ${M}_{13}$. First, a set of lowest-energy local minimum structures (as supported by vibrational analysis) were obtained by combining high-temperature first-principles molecular-dynamics simulation, structure crossover, and the selection of five well-known ${M}_{13}$ structures. Several new lower energy configurations were identified, e.g., ${\text{Pd}}_{13}$, ${\text{W}}_{13}$, ${\text{Pt}}_{13}$, etc., and previous known structures were confirmed by our calculations. Furthermore, the following trends were identified: (i) compact icosahedral-like forms at the beginning of each metal series, more opened structures such as hexagonal bilayerlike and double simple-cubic layers at the middle of each metal series, and structures with an increasing effective coordination number occur for large $d$ states occupation. (ii) For ${\text{Au}}_{13}$, we found that spin-orbit coupling favors the three-dimensional (3D) structures, i.e., a 3D structure is about 0.10 eV lower in energy than the lowest energy known two-dimensional configuration. (iii) The magnetic exchange interactions play an important role for particular systems such as Fe, Cr, and Mn. (iv) The analysis of the binding energy and average bond lengths show a paraboliclike shape as a function of the occupation of the $d$ states and hence, most of the properties can be explained by the chemistry picture of occupation of the bonding and antibonding states.

Density functional study of structural trends for late-transition-metal 13-atom clusters

Abstract: Because reactivity increases as particle size decreases and competition between numerous structures are possible, which affects catalytic and magnetic properties, we study the structural trends of late-transition-metal 13-atom clusters using density functional theory within the generalized gradient approximation to exchange-correlation functional. We consider open structural motifs, such as bilayer and cubic (recently found to have lower energy), and find new bilayer candidates that are even lower in energy. To study the influence of $d$-orbital filling on structural trends, we focus on Pt, Pd, and Rh clusters and find several new, low-energy structures for ${\mathrm{Pt}}_{13}$ and ${\mathrm{Pd}}_{13}$ from searches using a first-principle molecular dynamics high-temperature annealing. We find that 13-atom clusters prefer less square-cubic order as the $d$-orbitals gradually fill in the order of Rh, Pt, and Pd, and generally, low symmetry, open structures are preferred to high symmetry, compact ones, a trend explained from their electronic structures. For completeness, we briefly comment on improved exchange-correlation functionals affects on cluster morphology.