Joseph Muscat

On the prediction of band gaps from hybrid functional theory

Details of the band gap and band widths within materials are of fundamental importance to a wide range of applications. A hybrid scheme is used to predict the band gaps of a variety of materials. The electronic structure of silicon is examined in some detail and comparisons with alternative theories are made. Agreement with experimentally derived band gaps is at least as good as that obtained with sophisticated correlated calculations or perturbation theories. The functional is straightforward to implement, computationally efficient and produces ground state energy surfaces which are significantly more accurate than those computed using the best gradient corrected density functionals currently in use.

The influence of soft vibrational modes on our understanding of oxide surface structure

We examine the reasons for the poor quantitative agreement between the structures predicted from the minimum energy configuration of first principles calculations and those deduced from surface X-ray diffraction experiments for the structure properties of the TiO2(110) surface. In order to confine all numerical approximations very large scale all-electron first principles calculations are used. We find a very soft, anisotropic and anharmonic surface rigid-unit vibrational mode which involves displacements of the surface ions of approximately 0.15 A for thermal vibrations corresponding to room temperature. It is concluded that in order to perform an accurate comparison between theory and experiment for this and perhaps other oxide surfaces it will be necessary to take account of such anisotropic vibrations in models used to interpret experimental data. In addition the contribution of the vibrational entropy to the surface free energy is likely to be significant and must be taken into account when computing surface energies and structures.

Ab initio simulation of molecular processes on oxide surfaces

Ab initio calculations based on both density functional theory (DFT) and Hartree–Fock (HF) methods are used to investigate the energetics and equilibrium structure of the stoichiometric and reduced TiO2 (110) surface, the adsorption of potassium on the (100) surface and of water on the (110) surface. It is shown that DFT and HF predictions of the relaxed ionic positions at the stoichiometric surface agree well with each other and fairly well with recent X-ray diffraction measurements. The inclusion of spin polarisation is shown to have a major effect on the energetics of the reduced surface formed by removal of bridging oxygens. The gap states observed to be induced by reduction are not reproduced unless spin polarisation is included. Static and dynamic DFT calculations on adsorbed water on TiO2 (110) confirm that dissociation of H2O leads to stabilisation at low coverages, but suggest a more complex picture at monolayer coverage, in which there is a rather delicate balance between molecular and dissociated geometries.

A combined ab initio and photoelectron study of galena (PbS)

Abstract The results of a study of the physical, elastic and electronic properties of PbS using both first principles theoretical and experimental X-ray and ultraviolet photoelectron techniques are presented. We have investigated the influence of the computational approximations such as the basis set, pseudopotential and the treatment of exchange and correlation on the accuracy of the theoretical results. The current study work confirms that the binding energy of the S 3s state lies at 13 eV, in agreement with previous experimental work and contrary to earlier ab initio Hartree–Fock (HF) calculations. We establish that the reason for the discrepancy between previous experimental and theoretical determinations of the electronic structure of PbS is due to the treatment of the exchange interaction. We demonstrate that calculations using the HF approximation tend to overestimate the binding energies of the valence and core states yielding densities of states in poor agreement with experiment whereas calculations performed with density functional theory within either the local density or the generalised gradient approximation give a significantly better description of the electronic structure. Finally, we confirm that hybrid functional techniques such as B3LYP provide an excellent description of the physical and electronic structure of PbS.

First principles studies of the surface of galena PbS

Ab initio calculations of the surface structure and properties of the (001) surface of galena (PbS) have been performed using two complementary approaches. Both Gaussian and numerical basis functions have been utilized, as embodied within the programs CRYSTAL98 and SIESTA, with their associated implementations, in order to determine to what extent computational factors are influencing the results. Relaxations are presented based on calculations employing the HF, B3LYP, LDA, and GGA Hamiltonians. Significant relaxations (up to about 0.2 A) are found to occur at the surface which decay with increasing depth into the bulk. The magnitude of the relaxation oscillates between layers, leading to pairing, with Pb ions being displaced outwards (by approximately 0.05 A) relative to the S ions. We find that the LDA, GGA, and B3LYP Hamiltonians give results in reasonable agreement with each other, but that the structures computed with HF theory are vastly different. The present calculations also predict a surface core level shift of 0.3 eV for the sulphur 2p level at the surface itself, in excellent agreement with a recent experimental determination.

The calculation of structural, elastic and phase stability properties of minerals using first principles techniques: A comparison of HF, DFT and Hybrid functional treatments of exchange and correlation

We benchmark the performance of four treatments of electron exchange and correlation in the prediction of structural and elastic properties of a range of minerals. The treatments used are the Hartree-Fock (HF) theory, the local density approximation (LDA) and the generalised gradient approximation (GGA) to the density functional theory (DFT) and Beckes three parameter hybrid functional (B3LYP). We find that the hybrid functional, B3LYP method yields computed elastic properties in significantly better agreement to experiment than HF or DFT-LDA and performs at least as well, if not better than the most successful DFT-GGA functionals. We suggest that B3LYP is a simple, reliable and computationally efficient tool for the ab initio simulation of mineral systems.