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## Publications

Publications (98)

Yttrium copper di-selenide is a cheap compound having low toxicity with potential solar cell application. Understanding the thermal transport properties of YCuSe2 is crucial for the design of an efficient solar cell since it may provide insight on device lifetime and stability. However, no experimental data on the thermal and thermoelectric propert...

In the present study, we used density functional theory to study the cohesive and formation energies, mechanical and dynamical stabilities, optical properties and lattice thermal conductivity of trigonal bulk and monolayer PtS2. A careful investigation of the electronic band structure and density of states of the bulk and monolayer PtS2 shows that...

The structural, mechanical, dynamical stability, electronic and optical properties of orthorhombic perovskite CH3NH3SnI3 have been investigated using density functional theory (DFT) and many body perturbation theory calculations under pressure. Elastic parameters such as bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anis...

This study is the first attempt towards establishing computational insight into the structural, electronic, mechanical, dynamical and thermoelectric properties of the monoclinic phases of NaSbS2 and NaSbSe2. The mechanical properties are predicted using the Hill approximation. Dynamical stability was investigated by computing the phonon frequency t...

The present work is the first quantitative theoretical investigation of elastic, dynamical and thermodynamic properties of Zinc diselenide, ZnSe2 and Zinc ditelluride, ZnTe2 based on a density functional theory plane-wave pseudopotential method. The first-principles calculations were carried out using the local density approximation, (LDA) and the...

Using the Spectroscopic Limited Maximum Efficiency, and Shockley and Queisser predictor models, we compute the solar efficiency of the chalcopyrites AgMX2(M=In,Al;X=S,Se,Te). The results presented are based on the estimation of the electronic and optical properties obtained from first principles density functional theory as well as the many-body pe...

Using the spectroscopic limited maximum efficiency, and Shockley and Queisser predictor models, we compute the solar efficiency of the chalcopyrites AgMX 2 (M = In, Al; X = S, Se, Te). The results presented are based on the estimation of the electronic and optical properties obtained from first principles density functional theory as well as the ma...

The present study is the first attempt towards establishing computational insights into the structural, electronic, mechanical, dynamical and thermal properties of the tetragonal phases of potassium chalcoargentates (KAgX). We find that the lattice thermal conductivity of KAgX is anisotropic, with values of 0.553 (0.279), 0.509 (0.369) and 0.221 (0...

Mixed organometal perovskite CH3NH3PbI3 has recently emerged as a promising candidate for low cost, high-efficiency solar cells. The materials' thermoelectric properties which are key to the solar thermoelectric applications have not been investigated extensively. In this paper, the thermoelectric properties of organic-inorganic halide perovskite C...

Hybrid halide perovskites have recently emerged as new materials for solar cell applications leading to a new class of hybrid semiconductor photovoltaic cells, with very recent results demonstrating a 20.1% efficiency. Here, we investigate the structural stability, elastic constants, vibrational properties and lattice thermal conductivity of the or...

Oxide based thermoelectric materials have gained considerable interest due to their abundance, low toxic nature and high temperature stability. In the present work, we report the thermoelectric properties of α-MnO2 and β-MnO2 investigated using the Density Functional Theory with a Hubbard correction as implemented in the VASP code. Our calculated b...

We formulate a set of equations that facilitate an exact numerical solution of the Kohn-Sham potential for a finite Hubbard chain with nearest neighbour hopping and arbitrary site potentials. The approach relies on a mapping of the non-interacting Kohn-Sham ground state wave function onto the exact interacting system wavefunction and two interconne...

The structural, mechanical, electronic and thermoelectric properties of the low temperature orthorhombic perovskite phase of CH3NH3PbI3 have been investigated using density functional theory (DFT). Elastic parameters bulk modulus B, Young’s modulus E, shear modulus G, Poisson’s ratio ν and anisotropy value A have been calculated by the Voigt–Reuss–...

Materials science is arguably the most important discipline in the physical sciences that should be developed in Africa given its rich resources of minerals, energy and biological diversity. Developing materials science should therefore be an important goal for Africa with important opportunities for economic benefits and quality people development...

Sn clathrates are promising phonon glass, electron crystal materials (PGEC), in which the phonon free paths are short and the electron free paths are long. We analysed the relaxed structure of Sn clathrates using four different Density Funtional Exchange-Correlation functionals. The phonon structures were investigated as a first step in order to de...

Since the first efficient solid-state perovskite solar cells were reported in 2012, rapid development of the organic-inorganic hybrid halide perovskites has been made, and a new era in optoelectronic and solar cells technologies has emerged. The unique attributes of these hybrid halide perovskites make them highly promising materials for various pr...

Pyrites (FeS2) are solid minerals that are found abundantly in Nigeria and are easy to prepare in laboratories. In this work, FeS2 is studied extensively in its pure state as well as when iron is substitutionally doped with zinc and calcium at concentrations of 0, 0.25, 0.5, 0.75 and 1. Using density functional theory, the eectronic, dynamic and th...

In Kohn-Sham Density Functional Theory, the interacting system is mapped onto a fictitious independent particle system. In an ensemble continuous particle number formulation the exchange-correlation contribution to the potential of the independent particle system has a discontinuity as a function of particle number at integer particle numbers. This...

In Kohn-Sham Density Functional Theory, the interacting system is mapped onto a fictitious independent particle system. In an ensemble continuous particle number formulation the exchange-correlation contribution to the potential of the independent particle system has a discontinuity as a function of particle number at integer particle numbers. This...

Abstractauthoren Abstractauthoren The electronic and optical properties of semiconducting TiS3, HfS3, HfSe3, ZrS3, and ZrSe3 with structure P21/m have been investigated for the first time at the and BSE level of approximations, respectively. The structures were relaxed using PBE with the inclusion of van der Waal's correction terms to account for l...

The structural, electronic and optical properties of layered TcS2, TcSe2, and TcTe2 in the P structure have been investigated using different density functional (DFT) approximations with and without van der Waals corrections. The van der Waals correction terms are important for the accurate prediction of interlayer spacing in these layered structur...

We employ state-of-the-art ab initio density functional theory techniques to investigatethe structural, dynamical, mechanical stability and electronic properties of the ternaryAgInS2 compoundsunder pressure. Using cohesive energy and enthalpy, we found that from the six potentialphases explored, the chalcopyrite and the orthorhombic structures were...

During the phase two upgrade of the Large Hadron Collider (LHC), various components of the accelerator and ATLAS detector are due to be replaced or upgraded to withstand the increase in instantaneous luminosity. The minimum bias trigger scintillator (MBTS) plastics, located at 2.09 ≤ |η| ≤ 3.84, on the EndCaps of the Tile Calorimeter (TileCal) in t...

Using state-of-the-art Density Functional and Many Body Perturbation Theories, we study electronic and optical properties of the chalcopyrites AgAlX2(X=S,Se,Te). The Kohn-Sham Density Functional Theory (DFT) underestimates the fundamental and the optical gaps as a result of the particle number dependant discontinuity in the exchange-correlation pot...

We present first principles calculations of the electronic and optical properties of bulk and monolayer structures for four transition metal chalcogenides, MX2 (M=Zr and Hf; X=S, Se), using a post density functional many-body perturbation GW approximation in conjunction with the Bethe-Selpeter approximation (GW-BSE). Optical absorption spectra pred...

In this study we look at radiation damage and its adverse effects on plastic scintillators housed within the Tile Calorimeter (TileCal) of the ATLAS detector. The study focuses on determining how the interaction of ionizing radiation with plastic scintillators effects their efficacy and desired properties such as high light output and fast decay ti...

The extreme endothermic nature of silver nitride has been hampering its experimental characterization and there is incomplete detailed knowledge of the structural and physical properties of this material. Stimulated by this lack, we present a first-principles DFT and GW investigation of structural, electronic, and optical properties of bulk crystal...

First principles density functional theory (DFT) calculations for bulk structural, electronic and optical properties of ternary compounds AgAlX2 (X = S, Se, Te) were performed with two flavours of generalised gradient approximations (PBE and PBEsol) and the hybrid functional HSE06. Using cohesive energy as a stability criterion, we found that the c...

First principles density functional theory calculations of structural, thermal and electronic properties of the bulk chalcopyrites AlAgX2(X=S,Se,Te) were performed using the local density approximation and the modified Becke Johnson approximation. The optimized structure and lattice constants are obtained after a full relaxation of the structure wh...

Transition metal di-chalcogenide (TMDC) monolayers have potential applications in electronic and optical devices. Work on these atomically thin semiconductors is an exciting emerging eld of research. In this research the electronic and photo-absorption properties of monolayer ZrS 2 , ZrSe 2 , HfS 2 and HfSe 2 have been investigated using density fu...

We report the magnetic response of high purity type IIa single crystal CVD
diamond following high energy irradiation by 2.2 MeV proton micro-beams,
produced using Nuclear Microprobe techniques. Post-irradiation microanalysis is
achieved using Atomic Force, Magnetic Force and Electrostatic Force Microscopy
(AFM, MFM & EFM) as well as Raman spectrosc...

First principles density functional theory calculations of structural, thermal and electronic properties of the bulk chalcopyrites AlAgX2 (X=S,Se,Te) were performed using the local density approximation and the modified Becke Johnson approximation. The optimized structure and lattice constants are obtained after a full relaxation of the structure w...

The atomic and electronic structures of Pd3N, PdN and PdN2 were investigated
using ab initio density-functional theory (DFT). We studied cohesive energy vs.
volume equation of states (EOS) for a set of reported and hypothetical
structures. Obtained data was fitted to a third-order Birch-Murnaghan equation
of state (EOS) so as to identify the energe...

In the present work, the atomic and the electronic structures of Au3N, AuN
and AuN2 are investigated using first-principles density-functional theory
(DFT). We studied cohesive energy vs. volume data for a wide range of possible
structures of these nitrides. Obtained data was fitted to a Birch-Murnaghan
third-order equation of state (EOS) so as to...

Formal properties of ensemble density functionals are examined. Expressions for the difference between energy functionals where the particle number differs by one are constructed in terms of their first functional derivatives for the universal energy functional, the electron–electron repulsion energy functional, and the interacting kinetic energy f...

We present a detailed quantum mechanical non empirical DFT investigation of
the energy-optimized geometries, phase stabilities and electronic properties of
bulk Pt3N, PtN and PtN2 in a set of twenty different crystal structures.
Structural preferences for these three stoichiometries were analyzed and
equilibrium structural parameters were determine...

We present a first-principles investigation of structural, electronic and
optical properties of bulk crystalline Ag3N, AgN and AgN2 based on density
functional theory (DFT) and many-body perturbation theory. The equation of
state (EOS), energy-optimized geometries, cohesive and formation energies, and
bulk modulus and its pressure derivative of the...

We present a detailed first-principles DFT study of the equation of state
(EOS), energy-optimized geometries, phase stabilities and electronic properties
of bulk crystalline Cu3N, CuN and CuN2 in a set of twenty different structural
phases. We analyzed different structural preferences for these three
stoichiometries and determined their equilibrium...

Using the density functional adiabatic connection approach for an N-electron system it is shown that γ is the coupling constant that scales the electron–electron interaction strength. For the non-interacting Kohn–Sham Hamiltonian γ = 0 and for the fully interacting system γ = 1. is the Hartree plus exchange-correlation energy while f(r) and f(r) ar...

The atomic and electronic structures of PdN and PdN2 were investigated using ab initio density-functional theory (DFT). We studied cohesive energy vs. volume data for a set of reported and hypothetical structures. Obtained data was fitted to a third-order Birch-Murnaghan equation of state (EOS) so as to identify the energetically most stable phases...

The atomic and electronic structures of AuN, AuN2 and Au3N are investigated using first-principles density-functional theory (DFT). We studied cohesive energy vs. volume data for a wide range of possible structures of these nitrides. Obtained data was fitted to Birch-Murnaghan third-order equation of state (EOS) so as to identify the most likely ca...

Each Kohn-Sham eigenenergy can be mapped onto a difference between the interacting ground-state energies of successive integer-electron systems via a coupling-constant integration. Occupied Kohn-Sham energies can be mapped onto vertical ionization energies and virtual Kohn-Sham energies can be mapped onto vertical electron affinities. This mapping...

Some properties of exact ensemble density functionals can be determined by examining the particle number dependence of ground state ensemble density matrices for systems where the integer ground state energies satisfy a convexity condition. The results include the observation that the integral of the product of the functional derivative and Fukui f...

It is shown that the density-functional-theory exchange and correlation functionals satisfy 0=γEhx[ρN]+2Ecγ[ρN]−γEhx[ρN−1γ]−2Ecγ[ρN−1γ]+2∫d3r′[ρN−10(r)−ρN−1γ(r)]v0([ρN];r)+∫d3r′[ρN−10(r)−ρN−1γ(r)]r·∇v0([ρN];r)+∫d3r′ρN(r)r·∇vcγ([ρN];r)−∫d3r′ρN−1γ(r)r·∇vcγ([ρN−1γ];r)−∫d3r′fγ(r)r·∇vhxcγ([ρN];r)−2∫d3r′fγ(r)vhxcγ([ρN];r). In the derivation of this equat...

We use first-principles calculations based on density functional theory to study the structural properties and pressure-induced solid-solid phase transitions of ZnO. Both the local-density and the generalized gradient approximations are employed together with the projector augmented wave potentials to mimic the electron-ion interaction. We consider...

It is shown that the integral of the product of the charge density difference
of the N- and (N-1)-electron densities of the same Hamiltonian and the second
functional derivative of the Hartree plus exchange-correlation functional of
the N-electron density is equal to a constant.

It is shown that the electron density functional correlation functional
satisfies an equation that links the N-electron and (N-1)-electron densities of
the same adiabatically scaled Hamiltonian of the interacting electron system.

In electron density functional theory formal properties of density
functionals play an important role in constructing and testing approximate
functionals. In this paper it is shown that a set of density functionals
satisfy an equation that links functionals of densities of N and N-1 systems of
the same Hamiltonian. In particular the identity is sat...

The electron-electron mutual Coulomb repulsion energy density functional
satisfies an equation that links functionals and functional derivatives at
N-electron and (N-1)-electron densities for densities determined from the same
adiabatic scaled external potential for the N-electron system.

It is shown that the Hartree-plus-exchange-correlation density functional
potential for an integer N-electron system differs by a constant form the
corresponding potential for an (N-1)-electron system if the densities are
determined from the same external potential.

Bulk properties and stability of the entire series of group 4d transition metal carbides and nitrides are reported in this work. The theoretical calculations were carried out within Local Density Approximation and Generalized Gradient Approximation using the Perdew, Burke and Ernzerhof exchange correlation functional. The generalized gradient appro...

The optimized effective potential (OEP) equation can be used in a numerically efficient self-consistent form to solve for the density functional exchange and correlation potentials, as shown in a recent paper of Kummel and Perdew [Phys. Rev. Lett. 90, 43004 (2003)]. The uniqueness of an iterative solution of the OEP equation has not yet been adequa...

An alternative derivation of the exchange-only density-functional optimized effective potential equation is given. It is shown that the localized Hartree-Fock common energy denominator Green's function approximation (LHF-CEDA) for the density-functional exchange potential proposed independently by Della Sala and Görling [J. Chem. Phys. 115, 5718 (2...

The density-functional correlation potential of a finite system is shown to asymptotically approach a nonzero constant along a nodal surface of the energetically highest occupied orbital and zero everywhere else. This nonuniform asymptotic form of the correlation potential exactly cancels the nonuniform asymptotic behavior of the exact exchange pot...

It is shown that the product of the gradient of the density functional correlation potential and the charge density and the product of the gradient of the sum of the exchange and Hartree potentials and the charge density integrate to zero. The integral of the product of the charge density and the gradient of exchange potential is equal to an integr...

Parr and Liu [Chem. Phys. Lett., 276, 164 (1997)], showed that the electron-electron repulsion functional Vee[ρ] can be expressed in terms of its first and second functional derivatives under certain assumptions. Starting from this expansion it is shown that with the same assumptions, the density-functional exchange energy Ex[ρ], correlation energy...

The band-structure contribution to the force constants of an unrelaxed W(001) system is computed within a tight-binding approach. It is shown that the high density of surface states near the Fermi level gives rise to an enhanced attractive interaction between nearest neighbours on the surface. The attractive nearest-neighbour interaction drives a (...

Phonon spectra for Ta and W are calculated within a tight-binding approach. On the basis of these calculations the author shows that the (001) surface of Ta is stable while the W(001) surface is unstable with respect to distortions in the surface plane. The most unstable surface modes are in the vicinity of the M point, in agreement with experiment...

In order to improve approximations to density functionals such as the exchange-correlation functional, Exc[rho], and the independent Fermion kinetic energy functional, Ts[rho], it is essential to have information on the behavior of the exact functionals. Any approximation should then be made to satisfy these conditions. With this in mind, constrain...

The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Blöchl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and t...

The influence of many-body effects on the excitation spectrum of a binary alloy is investigated at the hand of a simple model within CPA. It is found that the detail of the spectrum can be significantly altered by many-body self-energy although the average behaviour does not differ drastically from the single-particle approximation.

The authors examine some properties of the electronic self-energy potential for metallic systems within the GW-plasmon-pole approximation. The self-energy is found to be highly state dependent. States with bonding character are depressed relative to states with less bonding character when compared to density functional eigenenergies, irrespective o...

The dielectric function for solid C60 is calculated within local-density-functional theory. The screened Coulomb interaction shows a remarkable orientation and position dependence. Inter-molecular screening is stronger than intra-molecular screening and screening increases with inter-molecular separation. Some of the inequivalent on-site atomic scr...

Coupling-constant integration of the mutual Coulomb interaction of a many-electron system formally gives the density functional exchange-correlation energy. The exchange-energy scales linearly with the coupling-constant, but the dependence of the correlation energy on the coupling-constant is not known. We present accurate numerical results for the...

Accurate functional derivatives of the kinetic part of the density functional correlation energy for two-electron systems are calculated numerically from the adiabatic coupling constant relation . The kinetic energy potential resembles a scaled mirror image of the correlation potential Numerical results show that the correlation energy functional a...

Coupling-constant integration of the mutual Coulomb interaction of a many-electron system formally gives the Density Functional exchange-correlation energy. The exchange-energy scales linearly with the coupling-constant, but the dependence of the correlation energy on the coupling-constant is not known. In this paper the correlation-energy is calcu...

An approximate non-local independent fermion kinetic-energy density functional is constructed from a model for the density functional exchange hole. The functional is exact for one- and two-electron systems and gives a good approximation for a homogeneous electron gas. Reasonable results are also obtained for model systems.

Translational and rotational invariance of functionals lead to hierarchies of equations between successive derivatives. These hierarchies allow alternating series expansions of some density functionals in terms of functional derivatives and charge density. Translational and rotational invariance also give rise to integrodifferential equations that...

Supercells consisting of 63 and 64 atoms are used to evaluate lattice relaxation at the vacancy in diamond using plane waves with soft and hard pseudopotentials. Car-Parrinello molecular dynamics calculations are undertaken allowing both nearest neighbour and second nearest neighbour atoms to move. Results are compared with static calculations wher...

Recently Joubert [Phys. Rev. A 54, 2479 (1996)] showed that the independent-fermion kinetic-energy kernel has a zero mode. In this Comment we remark that Joubert's main expression, which leads one to conclude that the contribution arising from the independent-fermion kinetic-energy functional to the local hardness is null, was previously deduced by...

Knowledge of the properties of the exchange-correlation functional in the form $\frac 1\lambda v_{xc}([\rho _\lambda ],\frac{{\bf r}}\lambda )$, where $\rho _\lambda ({\bf r})=$ $\lambda ^3\rho (\lambda {\bf r}),$ is important when expressing the exchange-correlation energy as a line integral $% E_{xc}[\rho ]=\int_0^1d\lambda \int d{\bf r}\frac 1\l...

From the minimal properties of the density functionals T-s[rho] and F[rho] under the constraint that the number of particles remains constant, it follows that (partial derivative(2)T(s)[rho])/[partial derivative rho(r)partial derivative rho(r')] and (partial derivative(2)F[rho])/[partial derivative rho(r)partial derivative rho(r')] are positive def...

Homogeneity and isotropy of space lead to two hierarchies of equations for the independent Fermion kinetic‐energy functional Ts[n]. The hierarchies link the mth functional derivative of Ts[n] to the (m+1)th functional derivative. The constraints imposed on Ts[n] are different for nonlocal functionals but equivalent for local functionals. As a conse...

The dielectric function for solid C60 is calculated within the RPA. The screened coulomb interaction shows a remarkable spatial and orientation dependence. Inter-molecular screening is considerably better than intra-molecular screening and inter-molecular screening increases with separation of molecules.

An expression for the static polarizability of a system of electrons moving in an external potential is obtained in terms of occupied wavefunctions of a single-particle Hamiltonian. The formalism is exact within the density functional theory. For a single electron in an external potential the polarizability is expressed in terms of the ground-state...

The band-structure contribution to the force constants of an unrelaxed W(001) system is computed within a tight-binding approach. It is shown that the high density of surface states near the Fermi level gives rise to an enhanced attractive interaction between nearest neighbours on the surface. The attractive nearest-neighbour interaction drives a (...

EXTENDED ABSTRACT. During the past few years it has been demonstrated that interatomic force constants and phonon spectra of transition metals can be adequately investigated on the basis of a tight-binding (TB) description of the electronic structure (ref. 1-5). The present method, similar to that used in (ref. 3,5), works entirely in real space, m...

Thesis (MSc)--University of Stellenbosch, 1982. Includes bibliographical references.

## Projects

Projects (3)

To identify potential transition metal oxides that have useful
thermoelectric properties using numerical simulation techniques

Find a better approximation of the correlation energy