Morse stretch potential charge equilibrium force field for ceramics: Application to the quartz-stishovite phase transition and to silica glass

To predict phase transitions in ceramics and minerals from molecular dynamics simulations, we have developed a force field in which the charges are allowed to readjust instantaneously to the atomic configurations. These charges are calculated using the charge equilibration (QEq) method. In addition to electrostatics, a two-body Morse stretch potential is included to account for short-range nonelectrostatic interactions. This MS-Q potential is applied herein to SiO_2, where we find that it describes well the fourfold coordinated and sixfold coordinated systems (such as quartz and stishovite), silica glass, and the pressure-induced phase transition from quartz to stishovite

Prediction of structures and properties for organic superconductors

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The main contributions of this thesis to the field of organic superconductors are basically (a) the band structure calculations for the investigations of the conduction properties of [...] using 2-D Hubbard Model with Unrestricted Hartree-Fock (UHF) theory, (b) ab initio quantum mechanical calculations for the structural characterizations and the properties of the donors of the organic superconductors, (c) electron-transfer boat-vibration (ET-BV) mechanism for the superconductivity of these materials, (d) developing force fields for BEDT-TTF and BEDT-TTF+. To provide a basis for understanding the puzzling electronic properties of the organic superconductor [...] (with Tc=10.4K), we carried out band calculations using the 2-D Hubbard Model with Unrestricted Hartree-Fock (UHF) theory. The electron transfer hopping interactions are from ab initio calculations and the Hubbard parameter (Uopt=0.678950 eV) is adjusted to fit Shubnikov-de Haas and magnetic breakdown experiments. The calculations lead to a two-band semi-metal with a momentum gap separating the electron and the hole bands. The anomalous experimental observations are explained in terms of BEDT-TTF related phonons coupling these two bands (lower temperature) and by anion related phonons (higher temperature). These results also provide a framework for describing the conduction properties of other such complexes. The donors of all known one- or two-dimensional organic superconductors, X, are based on a core organic molecule that is either tetrathiafulvalene (denoted as TTF) or tetraselenafulvalene (denoted as TSeF) or some mixture of these two molecules. Coupling X, with appropriate acceptors, Y, leads to superconductivity. The oxidized form of X may be X+ or X2+ species in the crystal. Using ab initio Hartree-Fock (HF) calculations (6-31G** basis set), we show that BEDT-TTF deforms to a boat structure (C2 symmetry) with an energy 28 meV (0.65 kcal/mol) lower than planar BEDT-TTF (D2 Symmetry). On the other hand BEDT-TTF+ is planar. Performing ab initio quantum mechanical calculations (HF/6-31G**) also on the other donors of organic superconductors, we find that all known organic superconductors involve an X that deforms to a boat structure while X+ is planar. This leads to a coupling between charge transfer and the boat deformation phonon modes. We propose that this electron-phonon coupling is responsible for the superconductivity and predict the isotope shifts [...] for experimental tests of the electron-transfer boat-vibration (ET-BV) mechanism. We suggest that new higher temperature organic donors can be sought by finding modifications that change the frequency and stability of this boat distortion mode. Based on this idea we have developed similar organic donors having the same properties and have suggested that with appropriate electron acceptors they will also lead to superconductivity. The highest transition temperature Tc organic superconductors all involve molecule BEDT-TTF coupled with an appropriate acceptor. The experimental structures exhibit considerable disorder in the outer rings and concomitant uncertainty in the structures of BEDT-TTF. We find that Hartree-Fock (6-31G** basis set) calculations leads to results within 0.01Å and 1° of experiment for the ordered regions allowing us to predict to composite structures expected to have this accuracy. We report optimized geometries and atomic charges for BEDT-TTF, BEDT-TTF+, and BEDT-TTF+1/2 that should be useful for atomistic simulations. The vibrational levels of BEDT-TTF and BEDT-TTF+ have been only partially observed and assigned. In order to provide a complete consistent description of all levels, we carried out HF calculations for all fundamental vibrational frequencies of BEDT-TTF and BEDT-TTF+ and obtained the Hessians for these structures. With these Hessians and available experimental frequencies, we developed the force fields for the neutral and cation BEDT-TTF molecules by using Hessian-biased method