Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set

We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

The construction of accurate pseudopotentials with good convergence properties for the first-row and transition elements is discussed. We show that by combining an improved description of the pseudowavefunction inside the cut-off radius with the concept of ultrasoft pseudopotentials introduced by Vanderbilt optimal compromise between transferability and plane-wave convergence can be achieved. With the new pseudopotentials, basis sets with no more than 75-100 plane waves per atom are sufficient to reproduce the results obtained with the most accurate norm-conserving pseudopotentials.

https://iopscience.iop.org/article/10.1088/0953-8984/6/40/015/pdf

Norm-conserving and ultrasoft pseudopotentials for first-row and transition elements

Preface Acknowledgements Notation Part I. Overview and Background Topics: 1. Introduction 2. Overview 3. Theoretical background 4. Periodic solids and electron bands 5. Uniform electron gas and simple metals Part II. Density Functional Theory: 6. Density functional theory: foundations 7. The Kohn-Sham ansatz 8. Functionals for exchange and correlation 9. Solving the Kohn-Sham equations Part III. Important Preliminaries on Atoms: 10. Electronic structure of atoms 11. Pseudopotentials Part IV. Determination of Electronic Structure, The Three Basic Methods: 12. Plane waves and grids: basics 13. Plane waves and grids: full calculations 14. Localized orbitals: tight binding 15. Localized orbitals: full calculations 16. Augmented functions: APW, KKR, MTO 17. Augmented functions: linear methods Part V. Predicting Properties of Matter from Electronic Structure - Recent Developments: 18. Quantum molecular dynamics (QMD) 19. Response functions: photons, magnons ... 20. Excitation spectra and optical properties 21. Wannier functions 22. Polarization, localization and Berry's phases 23. Locality and linear scaling O (N) methods 24. Where to find more Appendixes References Index.

Electronic Structure: Basic Theory and Practical Methods

During the past decade, computer simulations based on a quantum-mechanical description of the interactions between electrons and between electrons and atomic nuclei have developed an increasingly important impact on solid-state physics and chemistry and on materials science—promoting not only a deeper understanding, but also the possibility to contribute significantly to materials design for future technologies. This development is based on two important columns: (i) The improved description of electronic many-body effects within density-functional theory (DFT) and the upcoming post-DFT methods. (ii) The implementation of the new functionals and many-body techniques within highly efficient, stable, and versatile computer codes, which allow to exploit the potential of modern computer architectures. In this review, I discuss the implementation of various DFT functionals [local-density approximation (LDA), generalized gradient approximation (GGA), meta-GGA, hybrid functional mixing DFT, and exact (Hartree-Fock) exchange] and post-DFT approaches [DFT + U for strong electronic correlations in narrow bands, many-body perturbation theory (GW) for quasiparticle spectra, dynamical correlation effects via the adiabatic-connection fluctuation-dissipation theorem (AC-FDT)] in the Vienna ab initio simulation package VASP. VASP is a plane-wave all-electron code using the projector-augmented wave method to describe the electron-core interaction. The code uses fast iterative techniques for the diagonalization of the DFT Hamiltonian and allows to perform total-energy calculations and structural optimizations for systems with thousands of atoms and ab initio molecular dynamics simulations for ensembles with a few hundred atoms extending over several tens of ps. Applications in many different areas (structure and phase stability, mechanical and dynamical properties, liquids, glasses and quasicrystals, magnetism and magnetic nanostructures, semiconductors and insulators, surfaces, interfaces and thin films, chemical reactions, and catalysis) are reviewed. © 2008 Wiley Periodicals, Inc. J Comput Chem, 2008

Ab-initio simulations of materials using VASP: Density-functional theory and beyond.

Using a basis set of {similar to}3580 plane waves, we perform {ital ab} {ital initio} self-consistent calculations of the energy bands and cohesive energy of B{sub 12}C{sub 3}. Calculating stresses and forces, both the lattice constants and the positions of the atoms in the unit cell are determined. If trigonal symmetry is forced (i.e., all three carbons on the chain), the cohesive energy is 108.20 eV/(unit cell). In the experimentally observed structure with one boron on each chain and one carbon on each icosahedron, the cohesive energy is 109.48 eV/(unit cell). An indirect energy gap of 2.781 eV is obtained for this structure and charge-density--contour plots indicate that the ratio of the charge on the carbons to that on the borons is much greater than the 4:3 ratio of their valences.

Self-consistent calculations of the energy bands and bonding properties of B12C3.

We use micro- and nano-photoluminescence to study the temperature-dependent excitonic emission from CdSe quantum dots embedded in a ZnSe matrix. By varying the spatial resolution from 200 nm to 1.7 μm, we are able to study the temperature dependence of the ultranarrow (∼200 μeV) emission from excitons confined to single quantum dots, as well as statistical ensembles of up to 200 dots. By measuring the quenching of the photoluminescence (PL) with temperature, we find compelling evidence that the PL emission from these samples results from two different kinds of states. Similar to previous work, we find that a broad PL line persists to 300 K with an activation energy of ∼40 meV. However, we find that the ultranarrow lines are quenched at about 60 K, indicating an effective activation energy of only 4.0 meV.

Temperature-dependent micro-photoluminescence of individual CdSe self-assembled quantum dots

Statistical fluctuations of the magnetization are measured on the nanometer scale. As the experimental monitor we use the characteristic photoluminescence signal of a single electron-hole pair confined in one magnetic semiconductor quantum dot, which sensitively depends on the alignment of the magnetic ion spins. Quantitative access to statistical magnetic fluctuations is obtained by analyzing the linewidth broadening of the single dot emission. Our all-optical technique allows us to address a magnetic moment of only $\ensuremath{\approx}100{\ensuremath{\mu}}_{B}$ and to resolve statistical changes on the order of a few ${\ensuremath{\mu}}_{B}$.

Monitoring statistical magnetic fluctuations on the nanometer scale.

CdSe quantum dots in a Zn1−xMnxSe matrix: new effects due to the presence of Mn

Optical single dot studies in wide-band gap diluted magnetic CdSe/ZnMnSe quantum dots have been performed. Due to the sample design, the photoluminescence energy of the quantum dot signal is energetically below the internal Mn2+ transition, resulting in high quantum efficiencies comparable to nonmagnetic CdSe/ZnSe quantum dots. Magnetic-field- and temperature-dependent measurements on individual dots clearly demonstrate the exchange interaction between single excitons and individual Mn2+ ions, resulting in a giant Zeeman effect and a formation of quasi-zero-dimensional magnetic polarons.

Seongbok Lee

Papers

Self-consistent calculations of the energy bands and bonding properties of B12C3.

Temperature-dependent micro-photoluminescence of individual CdSe self-assembled quantum dots

Monitoring statistical magnetic fluctuations on the nanometer scale.

CdSe quantum dots in a Zn1−xMnxSe matrix: new effects due to the presence of Mn

Optical spectroscopy on individual CdSe/ZnMnSe quantum dots