My research lies in the field of computational electronic structure theory. In brief, it is the development of new theoretical methods, their implementatio into easy to use computer packages, and finally their application to novel scientific problems.
A major theme of my research has been the development of computational methods to interpret solid-state Nuclear Magnetic Resonance (NMR) experiments. Applications have included pharmaceutical compounds, (bio)minerals and glasses.
Further work has involved the development of techniques to interpret Electron Energy Loss Spectroscopy (EELS) with applications to nano-structured materials and interfaces.
Other work has focused on the use of so-called Wannier functions (www.wannier.org) to describe the properties of metallic systems: Fermi surface properties, transport, phase transitions etc.
This research has led to two publicly available codes: Castep www.castep.org for spectroscopic properties and Wannier90 www.wannier.org to obtain wannier functions.
An NMR crystallography investigation of furosemide.
Magn Reson Chem
This paper presents an NMR crystallography study of three polymorphs of furosemide. Experimental magic-angle spinning (MAS) solid-state NMR spectra are reported for form I of furosemide, and these are assigned using density-functional theory (DFT)-based gauge-including projector augmented wave (GIPAW) calculations. Focusing on the three known polymorphs, we examine the changes to the NMR parameters due to crystal packing effects. We use a recently developed formalism to visualise which regions are responsible for the chemical shielding of particular sites and hence understand the variation in NMR parameters between the three polymorphs.
NMR crystallography, solid-state NMR
Reducing the computational cost of NMR crystallography of organic powders at natural isotopic abundance with the help of 13 C-13 C dipolar couplings.
Magn Reson Chem
Structure determination of functional organic compounds remains a formidable challenge when the sample exists as a powder. Nuclear magnetic resonance crystallography approaches based on the comparison of experimental and Density Functional Theory (DFT)-computed 1 H chemical shifts have already demonstrated great potential for structure determination of organic powders, but limitations still persist. In this study, we discuss the possibility of using 13 C-13 C dipolar couplings quantified on powdered theophylline at natural isotopic abundance with the help of dynamic nuclear polarization, to realize a DFT-free, rapid screening of a pool of structures predicted by ab initio random structure search. We show that although 13 C-13 C dipolar couplings can identify structures possessing long range structural motifs and unit cell parameters close to those of the true structure, it must be complemented with other data to recover information about the presence and the chemical nature of the supramolecular interactions.
We propose modifications to the functional form of the Strongly Constrained and Appropriately Normed (SCAN) density functional to eliminate numerical instabilities. This is necessary to allow reliable, automatic generation of pseudopotentials (including projector augmented-wave potentials). The regularized SCAN is designed to match the original form very closely, and we show that its performance remains comparable.
Theory of momentum-resolved phonon spectroscopy in the electron microscope
From the inception of nuclear magnetic resonance as a spectroscopic technique, the local origin of chemical shifts has been a topic of discussion. A useful concept employed to describe it has been that of the "Lorentz sphere," the approximately spherical volume surrounding a given nucleus in which the electronic currents contribute significantly to the chemical shift, whereas the outside can be considered as an uniformly magnetised "bulk." In this paper, we use the output of the plane wave density functional theory code CASTEP to get a quantitative estimate of the Lorentz sphere in periodic systems. We outline a mathematical description of a radial buildup function for the magnetic shielding starting from the electronic currents and the simple assumption of periodicity. We provide an approximate upper bound for the Lorentz sphere's size in any crystal, then compute buildup functions for a number of sites in two molecular crystals, showing how various chemical features such as hydrogen bonds influence to convergence to the final shielding value.
Imaging the local electronic and magnetic properties of intrinsically phase separated Rb x Fe 2-y Se 2 superconductor using scanning microscopy techniques
Vibrational modes affect fundamental physical properties such as the conduction of sound and heat and can be sensitive to nano- and atomic-scale structure. Probing the momentum transfer dependence of vibrational modes provides a wealth of information about a materials system; however, experimental work has been limited to essentially bulk and averaged surface approaches or to small wave vectors. We demonstrate a combined experimental and theoretical methodology for nanoscale mapping of optical and acoustic phonons across the first Brillouin zone, in the electron microscope, probing a volume ~1010 to 1020 times smaller than that of comparable bulk and surface techniques. In combination with more conventional electron microscopy techniques, the presented methodology should allow for direct correlation of nanoscale vibrational mode dispersions with atomic-scale structure and chemistry.
Tin chemical shift anisotropy in tin dioxide: On ambiguity of CSA asymmetry derived from MAS spectra.
Solid State Nucl Magn Reson
Two different axial symmetries of the 119Sn chemical shift anisotropy (CSA) in tin dioxide with the asymmetry parameter (η) of 0 and 0.27 were reported previously based on the analysis of MAS NMR spectra. By analyzing the static powder pattern, we show that the 119Sn CSA is axially symmetric. A nearly axial symmetry and the principal axis system of the 119Sn chemical shift tensor in SnO2 were deduced from periodic scalar-relativistic density functional theory (DFT) calculations of NMR parameters. The implications of fast small-angle motions on CSA parameters were also considered, which could potentially lead to a CSA symmetry in disagreement with a crystal symmetry. Our analysis of experimental spectra using spectral simulations and iterative fittings showed that MAS spectra recorded at relatively high frequencies do not show sufficiently distinct features in order to distinguish CSAs with η ≈ 0 and η ≈ 0.4. The example of SnO2 shows that both the MAS lineshape and spinning sideband analyses may overestimate the η value by as much as ∼0.3 and ∼0.4, respectively. The results confirm that a static powder pattern must be analysed in order to improve the accuracy of the CSA asymmetry measurements. The measurements on SnO2 nanoparticles showed that the asymmetry parameter of the 119Sn CSA increases for nm-sized particles with a larger surface area compared to μm-sized particles. The increase of the η value for tin atoms near the surface in SnO2 was also confirmed by DFT calculations.
Asymmetry, Chemical shift, Chemical shift anisotropy, Nanoparticles, Oxides, Solid-state NMR, Structure