Although semilocal density functional approximations (DFA) are widely applied, none of them can capture the long-range van der Waals (vdW) attraction between the separated subsystems. However, they differ remarkably in the extent to which they capture intermediate-range vdW effects responsible for equilibrium bonds between neighboring small closed-shell subsystems. The local density approximation (LDA) often overestimates this effect, while the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) underestimates it. The strongly-constrained and appropriately normed (SCAN) meta-GGA often estimates it well. All of these semi-local functionals require an additive correction like non-local correlation functionals (vdW-DFs, VV10, rVV10 etc.) or empirical methods ( DFT+D3, DFT+vdW, DFT+XDM etc.) to capture the long-range part. The molecular complexes bonded by vdW interactions, layered materials, and molecule-surface interactions are a few examples of the systems where the long-range effects play a crucial role.In the first part of this assessment, we investigate the adsorption of benzene and thiophene over the (111) surface of copper (Cu), gold (Au), and silver (Ag). Thiophene and benzene are the prototypes of their respective classes of aromatic compounds and are the most widely studied molecules to model such systems. We first combine a non-local correlation functional (rVV10) with the various generalized gradient approximations (GGAs), namely PBE and PBEsol, along with the meta-GGAs SCAN and revSCAN (collectively known as base functionals), through a set of parameters obtained by fitting against the argon-dimer interaction energy curve. These parameters bridge the base functionals and rVV10 and guide the delicate balance between the short- and long-range interaction. We also utilize the recently introduced vdW-dZK model based on the theory of Zaremba and Kohn. It is a proven method to yield RPA-quality results for the physisorption of graphene over different metallic surfaces. We assess the adsorption energies, vertical adsorption distances, and the molecular-orientation at various sites and compare the results to the experimental values whenever available. Based on our calculations, the semilocal functionals alone underestimate the adsorption energies, reflecting the need for additional corrections. The rVV10-based methods generally bring the molecules closer to the surface and increase the binding energies. However, there is a discrepancy in the description of rVV10 based methods when the base functional is changed. While rVV10 combined with PBE slightly underestimates the adsorption energies, revSCAN+rVV10 and PBEsol+rVV10 are significantly overestimating. The methods PBE+vdW-dZK and SCAN+vdW-dZK, in general, predict better adsorption energies. In particular, SCAN+vdW-dZK stands out in predicting adsorption distances, adsorption energies, sites, and orientation closest to the experimental values whenever available. Apart from the inability of the semilocal DFAs to capture the long-range vdW interaction, they suffer from the so-called self-interaction error (SIE), in which an electron density incorrectly interacts with itself. At the semilocal level, the self-exchange-correlation energy can not counter-balance the self-Hartree energy, giving rise to the SIE problem. About 40 years ago, Perdew and Zunger proposed a solution to it by introducing a method that could remove the spurious SIE on an orbital-by-orbital basis. However, for size-consistency of this orbital-dependent theory, localized orbitals instead of delocalized Kohn-Sham orbitals are required. Recently, Pederson \textit{et al.} introduced an elegant scheme, known as Fermi-L\"owdin orbital self-interaction correction (FLOSIC), which could generate size-extensive and localized orbitals. For an exact functional, free from SIE, the negative of the highest occupied orbital (HO) eigenvalue would equal the first ionization energy (IE). In the second part of this assessment, we evaluate the HO eigenvalue of a representative test set containing 14 small to moderate-sized organic molecules using FLOSIC. The SIE inherent in the semilocal DFAs seriously underestimates the magnitude of the HO energy. Although LDA-SIC and PBE-SIC correct them, IEs are still significantly overestimated. A similar previous work by Vydrov \textit{et al.} reported the over-correction of PZ-SIC in many-electron regions, and various schemes with moderate success have been introduced since then to scale SIC down on those regions. Recently Zope \textit{et al.} introduced a method (LSIC) based on locally scaling-down PZ-SIC using an iso-orbital indicator (z$_\sigma$) which ensures that the correction is made only in the regions where they are required. We introduce a few other approaches similar to LSIC and demonstrate that these methods significantly improve the agreement between the calculated HO eigenvalues and experimental IEs of molecules. / Physics

This dissertation investigates how the magnetic properties of strontium hexaferrite change upon the substitution of foreign atoms at the Fe sites. Strontium hexaferrite, SrFe12O19 is a commonly used hard magnetic material and is produced in large quantities (around 500,000 tons per year). For different applications of strontium hexaferrite, its magnetic properties can be tuned by a proper substitution of the foreign atoms. Experimental screening for a proper substitution is a cost-intensive and time-consuming process, whereas computationally it can be done more efficiently. We used the ‘density functional theory’ a first principles based method to study substituted strontium hexaferrite. The site occupancies of the substituted atoms were estimated by calculating the substitution energies of different configurations. The formation probabilities of configurations were used to calculate the magnetic properties of substituted strontium hexaferrite. In the first study, Al-substituted strontium hexaferrite, SrFe12-xAl x O19, with x = 0.5 and x = 1.0 were investigated. It was found that at the annealing temperature the nonmagnetic Al+3 ions preferentially replace Fe+3 ions from the 12k and 2a sites. We found that the magnetization decreases and the magnetic anisotropy field increases as the fraction, x of the Al atoms increases. In the second study, SrFe12-x Gax O19 and SrFe12-x Inx O19 with x = 0.5 and x = 1.0 were investigated. In the case of SrFe12-x Gax O19, the sites where Ga+3 ions prefer to enter are: 12k, 2a, and 4f1. For SrFe12-x Inx O19, In+3 ions most likely to occupy the 12k, 4f1, and 4f2 sites. In both cases the magnetization was found to decrease slightly as the fraction of substituted atom increases. The magnetic anisotropy field increased for SrFe12-x Gax O19, and decreased for SrFe12-x Inx O19 as the concentration of substituted atoms increased. In the third study, 23 elements (M) were screened for their possible substitution in strontium hexaferrite, SrFe12-x Mx O19 with x = 0.5. In each case the site preference of the substituted atom and the magnetic properties were calculated. We found that Bi, Ge, Sb, Sn, and Sc can effectively increase the magnetization, and Cr, P, Co, Al, Ga, and Ti can increase the anisotropy field when substituted into strontium hexaferrite.

Covariant density functional theory (CDFT) is a modern theoretical tool for the description of nuclear structure physics. Here different physical properties of the ground and excited states in atomic nuclei have been investigated within the CDFT framework employing three major classes of the state-of-the-art covariant energy density functionals. The global performance of CEDFs for even-even nuclei are investigated and the systematic theoretical uncertainties are estimated within the set of four CEDFs in known regions of the nuclear chart and their propagation towards the neutron drip line. Large-scale axial relativistic Hartree-Bogoliubov (RHB) calculations are performed for even-even nuclei to calculate different ground state observabvles. The predictions for the two-neutron drip line are also compared in a systematic way with the non-relativistic results. CDFT has been applied for systematic study of extremely deformed, rotating N ∼ Z nuclei of the A ∼ 40 mass region. At spin zero such structures are located at high energies which prevents their experimental observation. The rotation acts as a tool to bring these exotic shapes down to the yrast line so that their observation could become possible with a future generation detectors such as GRETA or AGATA. The major physical observables of such structures, the underlying single-particle structure and the spins at which they become yrast or near yrast are defined. The search for the fingerprints of clusterization and molecular structures is performed and the configurations with such features are discussed. CDFT has been applied to study fission barriers of superheavy nuclei and related systematic theoretical uncertainties in the predictions of inner fission barrier heights in superheavy elements. Systematic uncertainties are substantial in superheavy elements and their behavior as a function of proton and neutron numbers contains a large random component. The benchmarking of the functionals to the experimental data on fission barriers in the actinides allows reduction of the systematic theoretical uncertainties for the inner fission barriers of unknown superheavy elements. However, even then they on average increase when moving away from the region where benchmarking has been performed.

To capture the underlying chemistry and physics of a system on electronic structure platform, it is necessary to accurately describe the intermolecular interactions such as repulsion, polarization, hydrogen bonding, and van der Waals interactions. Among these interactions, van der Waals (dispersion) interactions are weak in nature as compare to covalent bonds and hydrogen bonding, but it is physically and chemically very important in accurately predicting condensed phase properties such as Vapor liquid equilibria. This presents a significant challenge in modeling VLE using a first principles approach. However, recent developments in dispersion corrected (DFT-D3) and nonlocal density functionals can model dispersion interactions with reasonable accuracy. Here, we will present some of results that quantify efficacy of recent density functionals in predicting phase equilibria of molecular systems via first principle Monte Carlo (FPMC) simulations. Our aim is to assess the performance of several density functional by determining VLE, critical properties, dimer potential energy curves, vibrational spectra, and structural properties. The functional used in our study includes PBE-D3, BLYP-D3, rVV10, PBE0- D3, and M062X-D3. In addition, we have used the second order Møller-Plesset perturbation theory (MP2) method for computing density of argon at single temperature. The organic compounds considered for this study involves argon, CO2, SO2, and various hydroflurocarbons (R14, R134a, CF3H, CF2H2, CFH3) molecules. Additionally, the development of new materials, ionic liquids, and modification of industrial processes are an ongoing effort by researchers to efficiently capture acidic gases. Our ability to model these sorption processes using a first principles approach can have significant impact in speeding up the discovery process. In our work, we have predicted CO2 solubility in triethyl(butyl)phosphonium ionic liquid via FPMC simulations. Our results reveal the infrared spectra, structural and transport properties for pure ionic liquid and its mixture with CO2 through ab initio molecular dynamics simulations.

Atomistic modelling has widely been applied for studying structures and properties of materials. There are various methods to perform atomistic modelling. This master thesis presents a combined density functional theory (DFT) and cluster expansion (CE) study of Gd2O3 and Gd2O3-CeO2 interface (GCI) relevant for solid oxide fuel cells (SOFCs).    The energy differences (ΔE) of Va-O exchanges in C-type Gd2O3 and at GCI are calculated using both DFT and CE methods. We also calculated the migration energy (Emig) of Va jumps in Gd2O3 and at GCI by DFT. The comparison between the CE and DFT results demonstrates that the CE method provides a relatively accurate estimation of ΔE while it requires less computational resources. Furthermore, the CE method is used to study the Va migration in the vicinity of the Gd2O3-CeO2 interface. The potential energy landscapes of different types of paths are studied. / Atomistisk modellering har i stor utsträckning använts för att studera strukturer och  egenskaper  hos  material.  Det  finns  många  olika  metoder  för  att  utföra atomistisk   modellering.   Detta   masterprojekt   presenterar   en   kombinerad density functional theory (DFT) och   cluster expansion (CE) studie av Gd2O3- och Gd2O3-CeO2 gränssnittet (GCI), relevant för fastoxidbränsleceller (SOFC). Energiskillnaderna (ΔE) för Va-O-utbytet i C-typ Gd2O3 och vid GCI beräknas med   användning   av   både   DFT-   och   CE-metoder.   Vi   beräknade   också migrationsenergin   (Emig)   av   Va-hopp   i   Gd2O3   och   vid   GCI   med   DFT. Jämförelsen  mellan  CE  och  DFT-resultaten  visar  att  CE-metoden  ger  en relativt    noggrann    uppskattning    av ΔE    samt    att    den    kräver    mindre beräkningsresurser.   Vidare   används   CE-metoden   för   att   studera   Va- migrering  i   närheten   av   Gd2O3-CeO2-gränssnittet.   Det   potentiella   energilandskapet  för olika vägar studeras.

The growing interest of renewable fuel and energy sources has steadily increased over time due to climate changes. Research is being made around the world to find solutions for the different problems; one possible solution is to produce hydrogen gas to help phase out the usage of fossil fuels. So far, the technology for the hydrogen gas production is expensive for various reasons, one of the challenges is to minimize the energy usage for the production. Hydrogen could be used in fuel cells which can be used to fuel an electric car. In a fuel cell, hydrogen and oxygen gas are mixed to produce electrical energy as the main product, but it also forms thermal energy and water. Hydrogen gas can be produced from the reversed reaction; by electrolysis of water. This reaction requires energy and one way to minimize the energy usage for this is by using acatalyst. The goal with this master thesis was to see how the reaction rate of the oxygen evolution reaction can be affected by different single atom catalyst systems. The main structure for this catalyst in this thesis is aporphyrin molecule where different transition metals were tried as the active site. Different modifications on the structure were also made by exchanging some of the structures atoms and by adding different ligands.The purpose of this is to see how these modifications change the activity of the catalyst. The catalysts were optimized and calculated in a computational chemistry program called Gaussian 16. The calculations was made by using the DFT functional PBE0 and the basis sets Def2svp and Def2tzvpp. The results show that different modifications do affect the activity of the catalyst. The biggest variations in activity are from placing ligands under the active site while exchanging hydrogens to other substituents on the outer radial position can fine tune the results. The best active sites for this system came by using iridium, rhodium and cobalt which are all elements in group 9 of the periodic table. The lowest overpotential of 0.513 V was given by an iridium based system with four hydrogens exchanged by fluorides. / Runt om i världen finns ett ökat intresse för förnyelsebara energi och bränslekällor för att tackla klimat förändringarna. Stor del av forskningen som görs idag har i syfte att hitta nya lösningar för att minska klimatpåverkan i olika områden. Ett av forskningsområderna är hitta vägar till en miljövänligare vätgasproduktion där vätgasen skulle kunna användas i bränsleceller. Dessa celler kan sättas i elbilar och på så sätt fasa ut användingen av fossila bränslen. En av utmaningarna för vätgasproduktionen är att den idag är kostsam och kräver mycket energi. Forskare försöker hitta olika katalysatorer som kan minska energiåtgången som krävs vid elektrolys av vatten där syrgas och vätgas produceras. Målet med det här examensarbetet är att se hur en single atom catalyst kan påverka reaktionskinitiken för den syrgasbildande reaktionen vid elektrolys av vatten. Huvudstrukturen för katalysatorn som beräkningarna är gjorda på är en porphyrinmolekyl där olika övergångsmetaller kommer testas som det aktiva sätet i katalysatorn. Olika ligander kommer även tillsättas systemet samt utbyte av några väteatomer till olika substituenter i porfyrinstrukturen. Katalysatorn optimerades i det kvantkemiska beräkningsprogrammet Gaussian 16 med funktionalen PBE0 med basset Def2svp och Def2tzvpp. Resultaten visade att olika modifikationer på systemet hade en påverkan på katalysatorns aktivitet. Den största påverkan hade de olika liganderna som placerades under det aktiva sätet jämfört med de olika substituenterna. De bästa metallerna för katalysatorn var iridium, rhodium och kobolt vilket alla ligger i grupp nio i det periodiska systemet. Den lägsta överpotentialen på 0.513 V gavs av iridium systemet med fyra utbyta väten till fluor.

The goal of computational research in the fields of engineering, physics, chemistry or as a matter of fact in any field, is to study the properties of systems from the various principles available. In computational engineering, particularly in nano-scale simulations involving low-energy physics or chemistry, the goal is to model such structures and understand their properties from first principles or better known as \textit{Ab Initio} calculations. Geometry optimization is the basic component used in modeling molecules. The calculations involved are used to find the coordinates or the positions of the atoms of the molecule where it has the minimum energy and is hence stable. Efficient calculation of the forces acting on the atoms is the most important factor to be able to study the stable geometry of a molecule. In this thesis, the approach used begins with efficient electronic structure calculations using all electron calculations which paves the way for efficient force calculations. Kohn-Sham equations Density functional theory (DFT) are used to find the electron wave functions as accurately as possible using a finite element basis that introduces minimum errors in calculations. FEAST, a highly efficient density matrix based eigenvalue solver, is used to obtain accurate eigenvalues. Derivation of forces is done using the Hellmann-Feynman theorem. To find the minimum energy configuration of the system, Newton's iterative method is used that converges to the desired coordinates where the energy at the global minimum is found. The theory behind energy minimization and the calculations involved will be elaborated in this thesis and a method to move the atom in the existing framework will be discussed.

Many zeolites are deployed in various industrial processes owing to their robust catalytic performance and hydrothermal stability. Reactions in zeolites are catalyzed via framework aluminum. The Si/Al ratio is a metric that describes the relative aluminum content in zeolites. However, several researchers noted that the proximity of aluminum in the framework could impact the catalyst output [1–3]. In this work, the influence of paired acid sites is examined in ZSM-22. The 1-dimensional nature of ZSM-22 allows for direct assessment of aluminum proximity without the influence of channel intersection. Theoretical investigations via static density functional theory (DFT) optimization calculations on isolated and paired BAS in ZSM-22 revealed a potential increase in deprotonation potential energy (DPE), indicating a weaker acid with closer aluminum sites. One specific paired model, however, suggested stronger acid behavior, likely due to unfavorable proton-proton interactions influenced by proximity and orientation. Additionally, ammonia adsorption calculations inferred improved adsorption by isolated models, possibly due to unfavorable ammonium-proton interactions in the paired models. Reaction state calculations of ethylene and propylene oligomerization suggested enhanced stabilization of reactant molecules in paired sites. The synthesis of ZSM-22 showed sensitivity to precursor ratios and conditions, but pure samples were successfully achieved through iterative optimization. Catalytic testing of ethylene oligomerization with these samples, classified by their Si/Al ratios and unique fractions of paired acid sites, showed a correlation between higher fractions of paired BAS and increased catalytic activity and selectivity. Samples with higher fractions of paired BAS displayed a higher activity and selectivity for heavier hydrocarbons, explained by the enhanced adsorption capacity of paired BAS for larger reactant molecules, prompting further oligomerization and enhanced catalytic activity. Our findings demonstrate the impact of BAS proximity in dictating the activity and selectivity in ZSM-22 and provide valuable insights for designing more efficient industrial zeolite-based catalysts.

Thesis (Ph.D. (Physics)) -- University of Limpopo, 2017 / Density functional theory (DFT) study is employed in order to investigate the surfaces of, β-MnO2, β-TiO2 and β-VO2 (β-MO2) which act as catalysts in Li/Na-air batteries. Adsorption and co-adsorption of metal (Li/Na) and oxygen on (110) β-MO2 surface is investigated, which is important in the discharging and charging of Li/Na– air batteries. Due of the size of the supercell, and assuming that oxygen atoms occupy bulk-like positions around the surface metal atoms, only five values of (gamma) Γ are possible if constraint to a maximum of 1 monolayer (ML) of adatoms or vacancies: Γ= 0 surface is the stoichiometric surface, Γ= 1, 2 are the partially and totally oxidised surfaces, and Γ=-1, -2 are the partially and totally reduced surfaces. The manganyl, titanyl and vanadyl terminated surface is not the only surface that can be formed with Γ= +2. Oxygen can be adsorbed also as peroxo species (O2)2-, with less electron transfer from the surface vanadium atoms to the adatoms than in the case of manganyl and titanyl formation. The redox properties of the (110) surfaces are investigated by calculating the relative surface free energies of the non-stoichiometric compositions as a function of oxygen chemical potential. Increasing the temperature and lowering the pressure (i.e. more reducing conditions) we find the stoichiometric surface reduces first partially and then entirely at higher temperatures. The lithium orientation between two bridging oxygen and in-plane oxygen (bbi) orientation is much more stable for the three metal oxides, thus lithium generally prefers to adsorb where it will be triply coordinated to two bridging oxygens and one in-plane oxygen atom. However, sodium prefers to orientate itself on the bridging oxygen on the surface, but a triple coordination on sodium is also favourable. Oxygen adsorption on Li/MO2 was simulated and it was found that in all ii the metal oxides (MnO2, TiO2 and VO2) the most stable orientation is the dissociated composition where there is an oxygen atom on the “bulk-like” positions on top of each of the M cations. The surface lithium peroxide for MO2 simulated produces clusters with oxygen - oxygen bond lengths that are comparable to the calculated bulk and monomer discharge products reported in literature. Adsorption of oxygen on Na/MO2 was investigated and it was observed that the catalysts used encourage formation of the discharge product reported in literature, i.e. NaO2. The surface NaO2 appears to have comparable bond lengths to the calculated bulk and monomer NaO2. / National Research Foundation, South African Research Chair Initiative of the Department of Science Technology and Department of Energy storage Programme

Covalent SP3-hybridization defects in single-walled carbon nanotubes (CNTs) have been prevalent in recent experimental and theoretical studies for their interesting photophysical properties. These systems are able to act as excellent sources of single, infrared photons, even at room temperature, making them marketable for applications to sensing, telecommunications, and quantum information. This work was motivated by recent experimental studies on controllable defect placement and concentration as well as investigating carrier multiplication (CM) using DFT-based many-body perturbation theory (MBPT) methods to describe excitonic relaxation processes. We find that pristine CNTs do not yield appreciable MEG at the minimum threshold of twice the optical gap 2Eg, but covalent functionalization allows for improved MEG at the threshold. Finally, we see that defect-defect interactions within CNT systems can be modeled simply as HJ-aggregates in an effective Hamiltonian model, which is shown to be valid for certain, highly-redshifted defect configurations at low defect-defect separation lengths.