Vitek, Vaclav
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Publication Calculation of the Positions of the α- and β-bands in the Electronic Spectra of Benzenoid Hydrocarbons Using the Method of Limited Configuration Interaction(1963) Koutecký, Josef; Paldus, Josef; Vitek, VaclavThe positions of the α- and β-bands in the electronic absorption spectra of twenty aromatic benzenoid hydrocarbons were calculated by the semiempirical method of limited configuration interaction in the π-electron approximation using the Huckel molecular orbitals. The agreement of the experimental and calculated values is good for the β-band whereas a systematic deviation is observed for the α-band. This deviation cannot be removed by extending the configuration interaction of the monoexcited states constructed from the molecular orbitals considered. However, the consideration of electronic repulsion enables us to explain the character of the dependences of the experimental excitation energies on the excitation energies obtained by the simple Huckel method of molecular orbitals. Using a suitable choice of semiempirical parameters different for various electronic transitions (showing no large mutual differences) yields semiempirical interpolation formulas for the; p-, α-, and β-bands which give very good agreement with the corresponding experimental excitation energies for the compounds studied.Publication Monte Carlo Analysis of Stress-Directed Phase Segregation in Binary Thin Film Alloys Under Nonisothermal Annealing(2008-01-01) Nieves, Alex M; Vitek, Vaclav; Sinno, TalidThe use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential. We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomerdiffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.Publication Grain Boundary Structures in f.c.c, and b.c.c. Metals and Sites for Segregated Impurities(1979-07-01) Pond, R. C; Vitek, Vaclav; Smith, D. AThe key features leading to low-energy grain boundaries in metals are discussed by reference to computer-simulated structures and geometrical analysis in terms of atom packing. Low energy is found to be associated with boundary structures consisting of relatively dense packing, and this can in turn be expressed in terms of the space-filling packing of coordination polyhedra. The geometrical method of analysis is shown to be well suited to the identification of interfacial sites for segregated impurities.Publication Comment on "Glide Systems and Peierls Stresses in f.c.c. and b.c.c. Metals From Phonon Energies"(1974-07-01) Taylor, G.; Vitek, VaclavThe theory of Boffi et al. of Peierls stresses in crystals is criticized on physical grounds on a number of points.Publication Bond-Order Potential for Simulations of Extended Defects in Tungsten(2007-03-30) Mrovec, Matous; Gröger, Roman; Bailey, Aimee Gotway; Nguyen-Manh, Duc; Elsässer, Christian; Vitek, VaclavWe present a bond-order potential (BOP) for the bcc transition metal tungsten. The bond-order potentials are a real-space semiempirical scheme for the description of interatomic interactions based on the tight-binding approximation. In the hierarchy of atomic-scale-modeling methods the BOPs thus provide a direct bridge between electronic-structure and atomistic techniques. Two variants of the BOP were constructed and extensively tested against accurate first-principles methods in order to assess the potentials’ reliability and applicability. A comparison of the BOP with a central-force potential is used to demonstrate that a correct description of directional mixed covalent and metallic bonds is crucial for a successful and fully transferable model. The potentials are applied in studies of low-index surfaces, symmetrical tilt grain boundaries, and dislocations.Publication Many-Body Potentials and Atomic-Scale Relaxations in Noble-Metal Alloys(1990-05-15) Ackland, Graeme J; Vitek, VaclavWe derive empirical many-body potentials for noble-metal alloy systems in the framework of the Finnis-Sinclair model [Philos. Mag. A 50, 45 (1984)] which is based on a second-moment approximation to the tight-binding density of states for transition metals [F. Cyrot, J. Phys. Chem. Solids 29, 1235 (1968)]. The most important extension of the model is a simple incorporation of interspecies interactions which involves fitting the alloying energies. The importance of properly accounting for the local atomic relaxations when constructing the potentials is emphasized. The observed principal features of the phase diagrams of the alloys are all well reproduced by this scheme. Furthermore, reasonable concentration dependences of the alloy lattice parameter and elastic constants are obtained. This leads us to suggest that fine details of the electronic structure may be less important in determining atomic structures than are more global parameters such as atomic sizes and binding energies.Publication Analytic Environment-Dependent Tight-Binding Bond Integrals: Application to MoSi2(2000-11-06) Nguyen-Manh, Duc; Pettifor, David G; Vitek, VaclavWe present the first derivation of explicit analytic expressions for the environmental dependence of the σ, π, and δ bond integrals within the orthogonal two-center tight-binding approximation by using the recently developed bond-order potential theory to invert the nonorthogonality matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of the bond integrals between elemental bcc Mo and Si and binary C11b MoSi2 but also predicts the absence of any discontinuity between first and second nearest neighbors for the ddσ bond integral even though large discontinuities exist for ppσ, ppπ, and ddπ.Publication Atomistic Analysis of Phase Segregation Patterning in Binary Thin Films Using Applied Mechanical Fields(2010-03-02) Nieves, Alex M; Vitek, Vaclay; Sinno, TalidThe patterned compositional evolution in thin films of a binary alloy controlled by modulated stress fields is studied by employing Monte Carlo simulations. General features of stress-patterned phase segregation are probed using a binary Lennard-Jones potential in which the lattice misfit between the two components of the alloy is varied systematically. In general, patterning of the microstructure is found to be more robust in the low-mismatch binary systems because large lattice mismatch promotes plastic, and therefore, irreversible relaxation, during annealing. It is shown that some control over the relaxation process can be achieved by careful design of the applied thermal annealing history. Additional calculations have been performed using two other potentials for binary metallic systems, an embedded-atom method (EAM) potential for Cu–Ag and a modified embedded-atom method (MEAM) potential for Cu–Ni that represent examples of high and low-mismatched systems, respectively. The results obtained with generic Lennard-Jones potentials are in excellent agreement with those from the EAM and MEAM potentials suggesting that it is possible to derive general guidelines for accomplishing stress-patterned segregation in a variety of thin films of binary alloys.Publication Monte Carlo Analysis of Stress-Directed Phase Segregation in Binary Thin Film Alloys Under Nonisothermal Annealing(2008-11-14) Nieves, Alex M; Vitek, Vaclav; Sinno, TalidThe use of patterned stress fields to direct phase separation in thin film alloys is investigated computationally with Monte Carlo simulations in which atomic interactions are represented by a Lennard-Jones potential.We show that careful design of annealing schedules based on consideration of the system phase diagram can lead to vastly enhanced patterning kinetics. In particular, by avoiding the low temperature formation of highly stable nuclei within the entire system, the kinetics of patterning are accelerated by rapid monomer diffusion, rather than classical Ostwald ripening in which small precipitates must dissolve to feed larger ones.Publication Interatomic Forces and Atomic Structure of Grain Boundaries in Copper-Bismuth Alloys(1993-03-01) Yan, Min; Luzzi, David E; Šob, Mojmír; Vitek, Vaclav; Ackland, Graeme J; Methfessel, M.; Rodriguez, C. OThe many-body empirical potentials that describe atomic interactions in the copper-bismuth system were constructed using both experimental data and physical quantities obtained by ab initio full-potential linear muffin-tin orbital calculations for a metastable Cu3Bi compound. These potentials were then used to calculate the structure of a grain boundary in copper containing bismuth, which was at the same time studied by high-resolution electron microscopy (HREM). Excellent agreement between the calculated and observed structures is shown by comparing a through-focal series of observed and calculated images. This agreement validates the constructed potentials, which can be used with a high confidence to investigate the structure and properties of other grain boundaries in this alloy system. Furthermore, this study shows, that HREM combined with computer modeling employing realistic empirical potentials can decipher with great accuracy the structure of boundaries containing multiple atomic species.