|dc.description.abstract||In the past sixty years, X-ray, neutron and electron dffraction have emerged
as the structural techniques of choice in the solid state. However, despite many
advances in theory and instrumentation, these diffraction methods are still reliant
on a number of assumptions. Chief amongst these is that the atoms in the crystal
vibrate in a harmonic fashion.
This thesis is concerned with understanding the effects of anharmonic motion on
crystal structure determination and developing new ways of moving beyond the
harmonic approximation used in crystallography.
A method has been developed, using molecular dynamics simulations, to correct
experimental structures to equilibrium structures. This has been applied to
the crystal structures of phase-I deutero-ammonia, deutero-nitromethane and
benzophenone. Path-integral molecular dynamics simulations have been used
to obtain meaningful comparison with experimental data collected at low
temperatures. The simulations also offer information on the probability density
functions that describe thermal motion in solids. Using data from simulations of
nitromethane and other compounds it has been demonstrated that the molecular
dynamics-derived data can be used to assess and develop new functions for
modelling thermal motion in crystal structure refinements.
Finally, similar molecular dynamics techniques have been applied to determine
the equilibrium structures of some polyhedral oligomeric silsesquioxanes in the
gas phase. Some members of this class of compounds feature such strong
anharmonic motion that refinement of the structures using gas electron diffraction
is impossible without taking into account the effects of the anharmonicity.||en