Van der Waals interactions in DFT made easy by Wannier functions

Ubiquitous Van der Waals interactions between atoms and molecules are important for many molecular and solid structures. These systems are often studied from first principles using the Density Functional Theory (DFT). However, the commonly used DFT functionals fail to capture the essence of Van der Waals effects. Many attempts to correct for this problem have been proposed, which are not completely satisfactory because they are either very complex and computationally expensive or have a basic semiempirical character. We here describe a novel approach, based on the use of the Maximally-Localized Wannier functions, that appears to be promising, being simple, efficient, accurate, and transferable (charge polarization effects are naturally included). The results of test applications are presented.Comment: submitted to Phys. Rev. Let

Projector self-consistent DFT+U using non-orthogonal generalized Wannier functions

We present a formulation of the density-functional theory + Hubbard model (DFT+U) method that is self-consistent over the choice of Hubbard projectors used to define the correlated subspaces. In order to overcome the arbitrariness in this choice, we propose the use of non-orthogonal generalized Wannier functions (NGWFs) as projectors for the DFT+U correction. We iteratively refine these NGWF projectors and, hence, the DFT+U functional, such that the correlated subspaces are fully self-consistent with the DFT+U ground-state. We discuss the convergence characteristics of this algorithm and compare ground-state properties thus computed with those calculated using hydrogenic projectors. Our approach is implemented within, but not restricted to, a linear-scaling DFT framework, opening the path to DFT+U calculations on systems of unprecedented size.Comment: 4 pages, 3 figures. This version (v2) matches that accepted for Physical Review B Rapid Communications on 26th July 201

Optical properties of charged defects in monolayer MoS₂

We present theoretical calculations of the optical spectrum of monolayer MoS2 with a charged defect. In particular, we solve the Bethe–Salpeter equation based on an atomistic tight-binding model of the MoS2 electronic structure which allows calculations for large supercells. The defect is modelled as a point charge whose potential is screened by the MoS2 electrons. We find that the defect gives rise to new peaks in the optical spectrum approximately 100–200 meV below the first free exciton peak. These peaks arise from transitions involving in-gap bound states induced by the charged defect. Our findings are in good agreement with experimental measurements

Electronic structure of monolayer and bilayer black phosphorus with charged defects

We use an atomistic approach to study the electronic properties of monolayer and bilayer black phosphorus in the vicinity of a charged defect. In particular, we combine screened defect potentials obtained from first-principles linear response theory with large-scale tight-binding simulations to calculate the wave functions and energies of bound acceptor and donor states. As a consequence of the anisotropic band structure, the defect states in these systems form distorted hydrogenic orbitals with a different ordering from that in isotropic materials. For the monolayer, we study the dependence of the binding energies of charged adsorbates on the defect height and the dielectric constant of a substrate in an experimental setup. We also compare our results with an anisotropic effective mass model and find quantitative and qualitative differences when the charged defect is close to the black phosphorus or when the screening from the substrate is weak. For the bilayer, we compare results for charged adsorbates and charged intercalants and find that intercalants induce more prominent secondary peaks in the local density of states because they interact strongly with electronic states on both layers. These insights can be directly tested in scanning tunneling spectroscopy measurements and enable a detailed understanding of the role of Coulomb impurities in electronic devices

Investigation and technical comparison of new and conventional wheat combines performance for improvement and modification

Wheat is one of the most important food staff in consumption pattern of each country.  More than 50% of human energy is supplied from bread in developing countries.  Reducing of losses and waists in combine harvesting resulte increasing considerable wheat production in country.  Combine loss is less than 2%-3% in developed countries, while that of developing countries is about 15%-20% in different regions and circumstances of harvesting seasons and field conditions.  Regarding to research finding that maximum total combine losses are related to combine header loss, so it is necessary to determine precise losses and their causes to improve and modify existing combine's technical characteristics.  In this national research project that took place in three provinces of Tehran, Khorasan and Fars, effect of combine type and drum speed to grain losses and waists were investigated.  Experimental design was split plot in a completely randomized block design with three replications.  Duncan's test was used to statistical analysis of the means.  Combine type in four levels of New Holland TC 56, JD 1165, JD 955 and CLASS 68 as a main-plot and drum speed in three levels of 650, 750 and 850 rpm as a sub-plot.  Field experiments were carried out in the farmers field with two ha area. Different parameters and factors of each treatment were measured: 1. Pre-harvest loss, 2. Header loss, 3. Drum loss, 4. Cleaning loss, 5. Ground speed & combine field capacity, 6. Drum speed, 7. Linear drum speed, 8. Grain moisture content, 9. Cutting height.  The results of Tehran province show that New Holland TC 56 and JD 955 with 1.98% and 2.15% of the total combine losses are the lowest and positioned in class a while JD 1165 and CLASS 68 with 4.29% and 4.12% located in class b.  Also New Holland TC 560 has a highest combine capacity of 5187 kg/hr and gets a class of a while JD 1165 while CLASS 68 are in class b and JD 955 is in class c.  The results of Fars province show that the highest total combine loss was related to JD 955 and Class 68, 2.72% and 2.84%, respectively, and JD 1165 had the lowest total combine loss (1.61%).  The results of Khorasan province show that New Holland TC 56 had a lowest total combine loss (14.3 kg/ha) and got class a and JD 955, JD 1165 and CLASS 68 took class b, c and d, respectively.  Also New Holland TC 56 had a highest combine capacity (5,383.58 kg/hr) while CLASS 68, JD 955 and JD 1165 positioned in b, c and d classes, respectively.  Therefore, combines improvements and modifications are suggested as 1. Uniform cutting height, 2. compatible reel and ground speed, 3. using effective cutting width, 4. using proper drum speed, 5. choosing proper ground speed regarding crop density

An experimental study of permeability determination in the lab

Understanding the flow characteristics in laminar and turbulent flow regime is important for different aspects of reservoir and production engineering. One of the most important parameters in fluid flow is the permeability of the porous media. It is common practice in the industry to use Darcy and Forchheimers equations for characterising the fluid flow in the porous media at laminar and turbulent regimes, respectively. Core flooding experiments were performed with 1.5-inch diameter size core samples from limestone and sandstone formations. The permeability of the samples was measured in the laminar regime at basis flow rate. Then, the flow rate was increased in different steps and permeability was measured, accordingly. The plot of permeability versus flow rate was used to track the evolution of the flow regimes in the core porous media. There are different challenges to monitor the transition between laminar and turbulent regime through core flooding experiments. These challenges are discussed in this paper through both literature review and also experimental results. The results indicated that the core sample preparation, experiment control parameters and also test profiles are important aspects when measuring permeability in the lab. © 2012 WIT Press

Linear-scaling DFT+U with full local orbital optimization

We present an approach to the DFT+U method (Density Functional Theory + Hubbard model) within which the computational effort for calculation of ground state energies and forces scales linearly with system size. We employ a formulation of the Hubbard model using nonorthogonal projector functions to define the localized subspaces, and apply it to a local-orbital DFT method including in situ orbital optimization. The resulting approach thus combines linear-scaling and systematic variational convergence. We demonstrate the scaling of the method by applying it to nickel oxide nano-clusters with sizes exceeding 7,000 atoms.Comment: 10 pages, 4 figures. This version (v3) matches that accepted for Physical Review B on 30th January 201

The effect of chemistry and thermal fluctuations on charge injection barriers at aluminum/polyolefin interfaces

Charge injection at metal/polymer interfaces is a critical process in many technological devices, including high voltage capacitors and cables in which polyolefin materials, such as polyethylene (PE) and polypropylene (PP), are often used as insulation materials. We use simulations based on density-functional theory to study charge injection at aluminum/PE and aluminum/PP interfaces. Specifically, we investigate the influence of incorporating a variety of polar chemical impurities at the PE and PP chain ends on electron and hole injection barriers. Crucially, we account for the effect of thermal disorder by considering ensembles of thousands of interface structures obtained from ab initio molecular dynamics trajectories at 373 K. We show that the mean injection barrier can change by up to 1.1 eV for Al/PE and 0.6 eV for Al/PP, as compared to the pristine case, depending on which chemical impurity is introduced. We also show that the spread of injection barriers from thermal fluctuations also depends strongly on the chemistry of the impurity. The observed trends can be understood with a simple model based on thermal fluctuations of the dipole moment density associated with the chemical impurity at the interface. We further verify this model by considering larger interface models with lower impurity densities. Our results demonstrate that small chemical modifications, which may arise from oxidation, for example, have a significant influence on charge injection barriers in metal/polyolefin interfaces

Linear-scaling first-principles study of a quasicrystalline molecular material

Accepted versio

Linear-scaling density-functional simulations of charged point defects in Al2O3 using hierarchical sparse matrix algebra

We present calculations of formation energies of defects in an ionic solid (Al(2)O(3)) extrapolated to the dilute limit, corresponding to a simulation cell of infinite size. The large-scale calculations required for this extrapolation are enabled by developments in the approach to parallel sparse matrix algebra operations, which are central to linear-scaling density-functional theory calculations. The computational cost of manipulating sparse matrices, whose sizes are determined by the large number of basis functions present, is greatly improved with this new approach. We present details of the sparse algebra scheme implemented in the ONETEP code using hierarchical sparsity patterns, and demonstrate its use in calculations on a wide range of systems, involving thousands of atoms on hundreds to thousands of parallel processes