Calculations of Diffusion in FCC Binary Alloys Using On-the-fly Kinetic Monte Carlo

With the recent advancements in computer technologies and the improvements in simulation algorithms, along with the theories, has allowed scientists to carry out diffusion calculations more efficiently, especially for the cases where either the diffusion itself takes longer, or there is no available isotope to carry out the diffusion experiments such as tracer diffusion. In this work, we present a systematic approach to diffusion in intermetallic alloys such as weakly clustered Cu — Ni and weakly ordered Au — Ag. We use Accelerated Molecular Dynamics (AMD) combined with the Embedded Atom Method (EAM) to find the necessary saddlepoint energies. With this technique, we calculate the tracer diffusivity coefficients for Cu — Ni (temperature range 700 — 1300K) and for Au — Ag (temperature range 800 — 1300K) as a function of composition and temperature. We assume that the vacancy-assisted diffusion mechanism is governing the whole process. In our calculations we keep the vacancy concentration fixed. We observe that the results are in agreement with Arrhenius behavior as discussed in detail in Chapter 4: Results. However closer to critical temperature, the results are overwhelmed by statistical fluctuations. During the simulations at low temperatures, we sometimes find that the vacancy spends a large number of steps moving locally without accomplishing significant displacements or accumulating much simulated time. To overcome this \u27sandtrap\u27 problem, we develop a systematic approach which is discussed in Chapter 5: Sandtrap Limitation and How to Overcome it in detail. We also analyze the motion and the \u27width\u27 of the Anti-Phase Boundary (APB) perpendicular to its slip plane for Ni3Al using the same approach mentioned above for various temperatures (1000, 1200 and 1500K) under no driving force. We create the APB in the center of the sample and observe its motion. In addition, we create the APB farther away from the center and fix the atoms at the end of each side. This puts a bias on the motion of the APB by limiting the number of possible escapes in a certain direction, thus \u27forcing\u27 it to move towards the center. We observe the motion of the APB and the results are discussed in detail in Chapter 4: Results

Single metal catalysis: DFT and CAS modelling of species involved in the Fe cation assisted transformation of acetylene to benzene

Gas phase conversion of acetylene to benzene, assisted by a single metal cation such as Fe(+), Ru(+) and Rh(+), offers an attractive prospect for application of computational modelling techniques to catalytic processes. Gas phase processes are not complicated by environmental effects and the participation of a single metal atom is a significant simplification. Still the process is complex, owing to the possibility of several low-energy spin states and the abundance of alternative structures. By density functional theory modelling using recently developed models with range and dispersion corrections, we locate and characterise a number of extreme points on the FeC6H6(+) surface, some of which have not been described previously. These include eta-1, eta-2 and eta-3 complexes of Fe(+) with the C4H4 ring. We identify new FeC6H6(+) structures as well, which may be landmarks for the Fe(+)-catalysed production of benzene from acetylene. The Fe(+) benzene complex is the most stable species on the FeC6H6 cation surface. With the abundant energy of complexation available in the isolated gas phase species, detachment of the Fe(+) and production of benzene can be efficient. We address the issue raised by other investigators whether multi-configurational self-consistent field methods are essential to the proper description of these systems. We find that the relative energy of intrinsically multi-determinant doublets is strongly affected, but judge that the density functional theory (DFT) description provides more accurate estimates of energetics and a more plausible reaction path. [GRAPHICS]

Production of Carbamic Acid Dimer from Ammonia-Carbon Dioxide Ices: Matching Observed and Computed IR Spectra

The production of complex molecules in ammonia-carbon dioxide ices is presumed to pass through species of formula H3N:CO2 with further addition of ammonia and carbon dioxide. One possible landmark, carbamic acid, H2NCOOH, has been implicated among the products of warming and irradiation of such ices. Experimental study of the IR spectra of residues has suggested the presence of related species, including weakly bound 1:1 and 2:1 complexes of ammonia with carbon dioxide, zwitterionic carbamic acid, ammonium carbamate, and the dimer of carbamic acid. We computed the energetics and vibrational spectra of these species as well as the complex between ammonia and carbamic acid for gas and condensed phases. By means of a new spectrum-matching scoring between computed and observed vibrational spectra, we infer species that are most probably present. The leading candidates are ammonium carbamate, the carbamic acid-ammonia complex, and the carbamic acid dimer

Computational characterization of isomeric C4H2O systems: Thermochemistry, vibrational frequencies, and optical spectra for butatrienone, ethynyl ketene, butadiynol, and triafulvenone

Species of empirical formula C4H2O have been invoked either as elusive intermediates in flames or oxidations on heterogeneous catalysts, or as long-lived species in the interstellar medium. Butatrienone has been characterized experimentally, but isomers ethynyl ketene, butadiynol, and trifulvenone have been described only by computational modeling. Triafulvenone is of special interest as the ketene analog of the carbonyl compound cyclopropenone; both species contain seriously strained three-membered rings. In contrast to cyclopropenone, which is detected in the interstellar medium, triafulvenone continues to elude experimental capture. The contrast is attributed to a degree of aromatic stabilization in cyclopropenone and anti-aromatic destabilization in triafulvenone. In this report, we characterize the structure, vibrational and electronic spectra, and thermochemistry for triafulvenone and three of its isomers, butatrienone, ethynyl ketene, and butadiynol to assist experimental detection of these elusive species. Our calculations have shown that triafulvenone is the least stable of these four isomers; even the well-known butatrienone, is not the most stable. The so far undetected ethynyl ketene is thermodynamically the most stable of these isomers. To facilitate experimental detection of these species we provide vibrational frequencies calculated using both B3LYP/cc-pVTZ and MP2/cc-pVTZ level model chemistry corrected for anharmonicity including the possibility that the spectra may include overtones and combination bands for these species The regions of intense IR absorption and most important frequencies are also underlined for all the species involved. To guide the search for short-lived C4H2O species, we also characterize the optical spectrum. (c) 2015 Wiley Periodicals, Inc

Bonding Analysis of Compounds with Unusual Coordination of Carbon: Proposed Symmetric Systems with Six-Coordinate Carbon

The possibility of carbon tetravalence in geometries other than tetrahedral and of carbon hypervalence has been taken seriously since the 1970s. Computational modeling and subsequent experimental validation have established the existence of molecules with carbon atoms with planar tetravalence and as many as six objects in carbon's coordination sphere. In this work, we develop insight into the nature of bonding to carbon in these unusual environs as provided by Bader's Atoms in Molecules (AIM) analysis of the electron density, along with the electron localization function (ELF) and the non-covalent index (NCI). We review several well-established systems (spiropentadiene dication, hexamethyl benzene dication, dimethanospiro[2.2]octaplane dication, and 1,8-dimethoxy-9-dimethoxyanthracene cation) and propose new D-2d-symmetric variants of a hexacoordinated species

High-order harmonic generation from confined Rydberg atoms

We report results from our simulations of High Harmonic Generation (HHG) from a confined atom in a Rydberg state. We find that for the n = 2 excited state of H the cut-off of the harmonic spectrum is substantially extended compared to that for a free atom at the expense of the harmonic yield. This effect is dependent on the radius of the confining shell for a given n. We also observe that the confined spectrum exhibits cusps similar to those seen in the HHG spectra from ground state atoms in the presence of Cooper minima