Brief review related to the foundations of time-dependent density functional theory

The electron density n(\rb,t), which is the central tool of time-dependent density functional theory, is presently considered to be derivable from a one-body time-dependent potential V(\rb,t), via one-electron wave functions satisfying a time- dependent Schr\"{o}dinger equation. This is here related via a generalized equation of motion to a Dirac density matrix now involving $t$. Linear response theory is then surveyed, with a special emphasis on the question of causality with respect to the density dependence of the potential. Extraction of V(\rb,t) for solvable models is also proposed

Higher harmonics and ac transport from time dependent density functional theory

We report on dynamical quantum transport simulations for realistic molecular devices based on an approximate formulation of time-dependent Density Functional Theory with open boundary conditions. The method allows for the computation of various properties of junctions that are driven by alternating bias voltages. Besides the ac conductance for hexene connected to gold leads via thiol anchoring groups, we also investigate higher harmonics in the current for a benzenedithiol device. Comparison to a classical quasi-static model reveals that quantum effects may become important already for small ac bias and that the full dynamical simulations exhibit a much lower number of higher harmonics. Current rectification is also briefly discussed.Comment: submitted to J. Comp. Elec. (special issue

Implementation and benchmark of a long-range corrected functional in the density functional based tight-binding method

Bridging the gap between first principles methods and empirical schemes, the density functional based tight-binding method (DFTB) has become a versatile tool in predictive atomistic simulations over the past years. One of the major restrictions of this method is the limitation to local or gradient corrected exchange-correlation functionals. This excludes the important class of hybrid or long-range corrected functionals, which are advantageous in thermochemistry, as well as in the computation of vibrational, photoelectron and optical spectra. The present work provides a detailed account of the implementation of DFTB for a long-range corrected functional in generalized Kohn-Sham theory. We apply the method to a set of organic molecules and compare ionization potentials and electron affinities with the original DFTB method and higher level theory. The new scheme cures the significant overpolarization in electric fields found for local DFTB, which parallels the functional dependence in first principles density functional theory (DFT). At the same time the computational savings with respect to full DFT calculations are not compromised as evidenced by numerical benchmark data

Stacking dependence of carrier transport properties in multilayered black phosphorous

We present the effect of different stacking orders on carrier transport properties of multi-layer black phosphorous. We consider three different stacking orders AAA, ABA and ACA, with increasing number of layers (from 2 to 6 layers). We employ a hierarchical approach in density functional theory (DFT), with structural simulations performed with Generalized Gradient Approximation (GGA) and the bandstructure, carrier effective masses and optical properties evaluated with the Meta-Generalized Gradient Approximation (MGGA). The carrier transmission in the various black phosphorous sheets was carried out with the non-equilibrium Greens function (NEGF) approach. The results show that ACA stacking has the highest electron and hole transmission probabilities. The results show tunability for a wide range of band-gap, carrier effective masses and transmission with a great promise for lattice engineering (stacking order and layers) in black phosphorous.Comment: 18 Pages , 10 figure

Analysis of rotational coupling in collisions of Li+ with Ne leading to double excitation of Ne \ud

Electron angular distributions due to autoionization of Ne, doubly excited to the (2p43s2)1D state in collisions with Li+ in the energy range 1.2-2.2 keV, are measured in coincidence with Li+ scattered into a well defined direction ( Phi =0 degrees , Theta cm=10.8 degrees ). The experimental findings are analysed with the help of a collision model proposed earlier. In this model the initial excitation occurs by radial diabatic coupling to a molecular Sigma -state at small distances, followed by rotational coupling to Pi - and Delta -states at intermediate distances in the second half of the collision. The energy splitting between the Sigma -, Pi - and Delta -states is described by a model function. By adapting two parameters of this model function, the experimental findings can be reproduced within the experimental error in numerical calculations involving the relevant set of coupled differential equations. \u

Coins for blood and blood for coins: Towards a genealogy of sacrifice in the Transvaal Lowveld, 1930-1993

African Studies Seminar series. Paper presented 14 March 1994.In 1889 Tylor defined sacrifice as a form of gift-giving to spiritual beings (Tylor 1958:461). This definition has formed the conceptual framework for both classical and contemporary anthropological understandings of sacrifice. Subsequent theorists have viewed these gifts-to-gods as fundamentally different from gifts-to-people. Indeed, sacrifice has been seen as essential to the quest for morality, purity and salvation. The sacrificial gift, it has been contended, is offerred to establish communion with the sacred and to sacrilize the sacrificer (Robertson-Smith 1984, Hubert and Mauss 1964). Evans-Pritchard (1970) described Nuer sacrifices as aimed at ridding individuals from the dangerous state of nueer brought about by pathogenic contact with the Divinity. More recently, Gregory (1980) and Parry (1986) have viewed sacrifice as a 'pure gift' which is surrendered without the expectation of recall and return in this life. Parry (1986: 460-61) suggests that in salvation-orientated religions ultimate destiny is determined by supernatural reward. As such unreciprocated gifts mean liberation from the bondage of the profane world and create an image of the other world in this one. Whilst these contributions yield general insight, there is an urgent need to refocus attention on the meanings of sacrifice in particular performative and narrative contexts (Hoskins 1993). Generalized and static formula often obscure intricate, diverse, and changing emic models of sacrifice. This article investigates continuities and changes in concepts of sacrifice among Sotho and Tsonga-speakers of the Transvaal lowveld. It warns against the generalized formulation of sacrifice as a moralistic act and contemplates the possibility that sacrifice may well be motivated by more imediate and materialistic concerns. My analysis is based on fieldwork I have conducted, for intermittent periods over past four years, in the lowveld village of Green Valley. Green Valley has a population of approximately 20 000 people. It forms part of the Setlhare chiefdom and is situated in the Mapulaneng area of Lebowa. My account is diachronic, but genealogical rather than historical. The aim is not to provide a comprehensive overview of process based on the chronology of events, but merely to highlight prominent cultural assumptions underlying the conceptions of sacrifice at different times [1]. Along with Cousins and Hussain (1984) I see the critical potential of the genelogical method as lying in its ability to trance symbolic connections between seemingly diverse phenomena. As such phenomena can be shown as not nearly as timeless and distinct as they appear. My analysis differs significantly from earlier functionalist and structuralist accounts of sacrifice in the lowveld (Junod 1966, Krige and Krige 1965, Monnig 1988, Hammond- Tooke 1981, de Heusch 1985). I do not merely focus on the performative aspects of offerings to the ancestors, but incorporate reference to mythical and imagined sacrifices. As such I explore the wider meanings of sacrifice in cultural discourses. These discourses are located within changing local religious, socio-political and economic contexts

Effect of line defects on the electrical transport properties of monolayer MoS2_{2} sheet

We present a computational study on the impact of line defects on the electronic properties of monolayer MoS2. Four different kinds of line defects with Mo and S as the bridging atoms, consistent with recent theoretical and experimental observations are considered herein. We employ the density functional tight-binding (DFTB) method with a Slater-Koster type DFTB-CP2K basis set for evaluating the material properties of perfect and the various defective MoS2 sheets. The transmission spectra is computed with a DFTB-Non-Equilibrium Greens Function (NEGF) formalism. We also perform a detailed analysis of the carrier transmission pathways under a small bias and investigate the phase shifts in the transmission eigenstates of the defective MoS2 sheets. Our simulations show a 2-4 folds decrease in carrier conductance of MoS2 sheets in the presence of line defects as compared to that for the perfect sheet

Relocation in Tseki & Phuthaditjhaba: A comparative ethnography of planned & unplanned removals in Qwaqwa

African Studies Seminar series. Paper presented August 1989.The dominant theme in the recent history of Qwaqwa has been the relocation of thousands of people from "common" South Africa into the "homeland". In 1929 Ross (1930: 88) estimated that the population of Witzieshoek (as Qwaqwa was then known) was 8000 and wrote: In the past few years many folk who could make no existence on the farms outside have moved in. They are the old folk and the poor. This would have to be prohibited because Witzieshoek is already full, or Witzieshoek would have to be enlarged by the addition of nearby farms. (My translation). Subsequent estimates of Qwaqwa's population are: 23 860 in 1970 (BENSO 1978: 9), at least 300 000 in 1980 (Murray 1985: 289) and between 400 000 and 500 000 people in 1982 (Sharp 1982: 13). These figures are suggestive of mass population relocations It is generally possible to distinguish three sending areas. Murray (1985:290) estimates that perhaps 60% of people relocated to Qwaqwa came off white-owned farms, particularly in the northern, eastern and northwestern districts of the Orange Free State (OFS). The remaining 40%, he suggests, came from towns such as Harrismith, Bethlehem, Senekal, Vereeniging, Klerksdorp and Bloemfontein. A relatively small number of people have also been relocated from other "homelands"…. This paper is particularly concerned with highlighting the diverse implications of relocation for different sections of Qwaqwa's population. Despite the conceptual difficulties involved the the concepts "forced" and "voluntary", it is my intention to show that such distinctions may indeed be valuable in understanding relocation processes. It is argued that "official" removals imply at least some prior planning by the state to cater for the needs of those affected. Evidence indicates that people subject to forced removals from OFS townships by the state have enjoyed favourable access to resources and amenities in the 'homeland'. Such people are highly noticeable and taken account of by government and administration in the subsequent allocation of services in the 'homeland'. By contrast, former inhabitants OFS rural areas who have unofficially made their way into the 'homeland's' closer settlements have been severely disadvantaged. Their removals were unplanned and no administrative provisions had been made to meet even their most basic needs. Subsequently they have been largely unnoticed in the allocation of services. This paper reports the results of fieldwork undertaken in the Apollo (plural Diapollo) housing section in the town of Phuthaditjhaba (from August 1984 until February 1985) and the closer settlement of Tseki (in April, June-July and December 1983). The housing section was opened for settlement in March 1971

Witches, mysteries, rumours, dreams and bones: Tensions in the subjective reality of witchcraft in the Mpumalanga lowveld, South Africa

African Studies Seminar series. Paper presented 10 March 1997Evans-Pritchard's classical text Witchcraft, Oracles and Magic Among the Azande (1937) lay the foundations for contemporary scholarly understandings of witchcraft. Yet the author's central contention that witchcraft presents a logical explanation for misfortune has been less inspirational than his suggestion than that witchcraft accusations express regularly recurring socio-structural conflicts [2]. This idea was developed most fully by Marwick (1970) who argued that witchcraft accusations present a social "strain-gauge". This formulation is based on two closely related assumptions. First, that at a general level, the distribution of witchcraft accusations, between persons standing in various relationships, reveals tension points in the social structure. Anthropologists and historians have contended that witchcraft accusations indicate different sorts of tensions in different social contexts. Witchcraft accusations have been shown to cluster between different matrilineal segments among the Chewa of Northern Rhodesia (Marwick 1965), agnates and affines among the Zulu of South Africa (Gluckman I960), youths and elders among the Gisu of Uganda (Heald 1986), competing work parties among the Hewa of New Guinea (Steadman 1985), commoners and new state elites in Cameroon (Geschiere 1988), and between men and women in colonial Peru (Silverbladt 1987). Second, the social strain hypothesis assumes that tense relations are the prime determinants of whom the accused shall be. For example, Macfarlane argues that in sixteenth century Essex witchcraft accusations arose from quarrels over gifts and loans, rather than strange events. This article critically reexamines the relationship between social tensions and witchcraft. It draws on fieldwork conducted between 1990 and 1995 in Green Valley, a village situated in the lowveld of Mpumalanga, South Africa. In 1991 Green Valley had a population of approximately 20 000 Northern Sotho and Tsonga-speakers [4]. In the article I aim to focus on how individuals subjectively inferred the existence of witchcraft and the identity of alleged witches, rather than to explore the quantitative distribution of witchcraft accusations. From this perspective, I suggest that social tensions by themselves are less accurate predictors of witchcraft attributions and accusations than the literature may lead us to believe. The article is divided into two parts. The first considers the ontological status of witchcraft in local knowledge. I argue that the perception of witchcraft as a transcendent reality immunizes the belief against disproof. Yet in specific situations the occurrence of mysterious events, circumstantial evidence, revelations through divination and dreams, and confessions attested to the reality of witchcraft. Part two provides a detailed analysis of five case studies, and critically scrutinizes the role of social tensions relative to other types of evidence. I argue that social tensions were neither a sufficient, nor even a necessary, condition for witchcraft accusations. Villagers did perceive a conflictual relationship between the victim and the accused, prior to the advent of misfortune, as a motive for witchcraft. Tensions were therefore part of the wider framework of evidence they used to justify particular accusations. But villagers believed that witches often struck without motive

Towards a simplified description of thermoelectric materials: Accuracy of approximate density functional theory for phonon dispersions

We calculate the phonon-dispersion relations of several two-dimensional materials and diamond using the density-functional based tight-binding approach (DFTB). Our goal is to verify if this numerically efficient method provides sufficiently accurate phonon frequencies and group velocities to compute reliable thermoelectric properties. To this end, the results are compared to available DFT results and experimental data. To quantify the accuracy for a given band, a descriptor is introduced that summarizes contributions to the lattice conductivity that are available already in the harmonic approximation. We find that the DFTB predictions depend strongly on the employed repulsive pair-potentials, which are an important prerequisite of this method. For carbon-based materials, accurate pair-potentials are identified and lead to errors of the descriptor that are of the same order as differences between different local and semi-local DFT approaches