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 Cerius2 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 NH4Cl,[14] the structure and compressibility of garnets,[5,6,9,16] the structure of Cu6PbO8[13] and the structure and properties of CsHSO3.[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 CNx 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]