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Past Research

*Electron Energy Loss Spectroscopy
(EELS)* My thesis work involved the development of a new
scheme for the calculation of Energy Loss Near Edge Structure (ELNES) within
the framework of Density Functional Theory (DFT).[19,22,23,26]
Excellent agreement was found with experiment for the K-edges of graphite,
diamond and boron nitride. The use of the plane wave pseudopotential approach
offers the possibility of very large scale calculations, especially in
combination with parallel supercomputers.
*Linear optical properties*
To complement the calculation of ELNES, linear optical properties
were calculated through the complex dielectric function and it was implemented
within the same plane wave pseudopotential single particle approach.
*Population analysis*
Projecting electronic wavefunctions, expressed in terms of plane waves,
onto a localised atomic basis set allows a direct interpretation of the
electronic structure in terms of atomic charges, bond population or bond
orders and partial densities of states.[24,25]
Our method is integrated within the MSI Cerius^{2} interface to
CASTEP and is now widely used, allowing an interpretation more based on
chemical intuition.
**k.p** perturbation theory This
well known technique for the calculation of electronic band gradients and
curvatures has been extended to the case of both norm conserving and ultrasoft
non-local pseudopotentials allowing its use in modern electronic structure
calculations.[11]
*Brillouin zone integration*
An extrapolative approach to efficient Brillouin zone integration was developed
which avoids problems associated with band crossings.[20]
Gradients and curvatures calculated using **k.p** perturbation theory
were used to produce a piecewise quadratic representation of the electronic
band structure.
*Ultrasoft pseudopotentials*
I took part in a project to integrate the highly efficient and accurate
Vanderbilt ultrasoft pseudopotentials into the CASTEP code. I extended
the range of the pseudopotentials to cover the *f*-electron elements,
and implemented the calculation of stress tensors within the Vanderbilt
formalism, essential if complex, low symmetry, structures are to be relaxed.
At the same time, we developed a database of pseudopotentials to cover
the entire periodic table.[12]
*Cleavage of Diamond*
We calculated the theoretical strength of diamond in the <100>, <110>
and <111> directions from first principles. We were then able to explain
the remarkable dominance of the {111} cleavage plane when diamond is fractured.[15]
*Complex inorganic solids*
We have studied structure-property relationships in complex, low symmetry,
inorganic solids, recently investigating domain walls in NH_{4}Cl,[14]
the structure and compressibility of garnets,[5,6,9,16]
the structure of Cu_{6}PbO_{8}[13]
and the structure and properties of CsHSO_{3}.[21]
*Systematic prediction of crystal
structures* A generally applicable and systematic prediction
of crystal structures and their properties is an important goal of crystallography
and materials science. We have developed such a general and systematic
approach. This approach is based on a combination of graph theory with
quantum mechanics. As an application, structures, properties and relative
stabilities of small hypothetical carbon polymorphs have been investigated.[8,17]
*New CASTEP* I am
part of the seven strong UK based CASTEP Developers Group. We are currently
engaged in a complete rewrite of the CASTEP code -- the original being
unmaintainable. We have completed the design stage (which resulted in a
specification document for the project), and are now part way through the
coding. I am personally responsible for the ultrasoft pseudopotentials,
the I/O module, and the calculation of forces and stresses.
*``Hard'' Carbon Nitride compounds*
We have investigated the possible structures of CN_{x} compounds,
and their expected properties.[10] The
collaboration has now extended to the calculation of the electronic properties
of carbon nanotubes[1]. My extrapolative
Brillouin zone integration scheme is well suited to this, and the densities
of states so calculated exhibit the clear van Hove singularities expected
for such two dimensional electronic systems.
*Structural properties of lanthanides
and actinides* We have shown that the structural properties
of *f*-electron compounds can be calculated on the same footing as
the rest of the periodic table within the ultrasoft pseudopotential approximation.[7]