Harmonic vibrational effects on isotropic hyperfine coupling constants molecular hydrogen in interstellar ice and atomic relaxation in doped silicon

Three separate topics are presented covering the temperature dependence of isotropic hyperfine coupling constants, an anharmonic analysis of molecular hydrogen in interstellar ice and the microscopic structure of doped crystalline silicon lattices.;The temperature dependence and vibrational effects of isotropic hyperfine coupling constants are studied using the ab-initio density functional methods BLYP and B3LYP with the common double and triple zeta basis sets 6-31G(d,p) and 6-311(d,p). Harmonic oscillator wavefunction averages for all normal modes of a molecule are accounted for and the temperature dependence is developed from the Boltzmann distribution.;An ab-initio study of the atomic displacements and microscopic structure of phosphorus and arsenic in doped silicon is carried out. The structures and relative displacements are estimated using SCF, DFT and MP2 methods with STO-3G, 3-21G, 3-21G(d,p) and 6-31G(d,p) basis sets with a 13 atom silicon molecular cluster. The calculations are carried out on clusters using empirical parameters for Si-Si and Si-H bond lengths in addition in fully optimised clusters.;Following the discovery of a species thought to be molecular hydrogen in interstellar clouds by IR spectroscopy, ab-initio density functional B3LYP calculations are performed to back up experimental and semi-empirical studies of molecular hydrogen in amorphous ice. Anharmonic calculations are carried out on hydrogen molecules adsorbed onto ice and trapped in a cage. The issue of species other than molecular hydrogen and surface transport of hydrogen is addressed.