Basics of density-functional theory and fast guide to actual calculations Matthias Scheffler
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1 Basics of density-functional theory and fast guide to actual calculations Matthias Scheffler I. From the many-particle problem to the Kohn-Sham functional II. From the total energy to materials science and biophysics "the ab initio line of computational sciences and engineering" Modeling Materials and Bio-Molecular Properties and Functions: The Many-Body Schrödinger Equation With: 1, w3.rz-berlin.mpg.de/ ~michaeli/member/ MaterialsScienceLectures/ k pdf 1
2 Born-Oppenheimer Approximation Where Φ ν are solutions of the electronic Hamiltonian : ({r k }) ({r k }) = ({r k }) frequently (commonly) applied approximations: neglect non-adiabatic coupling (terms of order m/m I ) keep onlyλ 0 electronic and nuclear degrees of freedom decouple Some Limits of the Born-Oppenheimer Approximation It does not account for correlated dynamics of ions and electrons. For example: - polaron-induced superconductivity - dynamical Jahn-Teller effect - some phenomena of diffusion in solids - non-adiabaticity in molecule-surface scattering - etc. 2
3 Wave-Function Theories Restrict the study to a selected subclass of functions Φ (Hartree and Hartree-Fock theory); or Quantum Monte Carlo. The Hohenberg-Kohn Theorem (1964) n(r) = n[φ] N = <Φ δ(r r i ) Φ> i The set of non-degenerate ground state wave functions Φ of arbitrary N-particle Hamiltonians. The set of particle densities n(r) belonging to nondegenerate ground states of the N-particle problem. The dashed arrow is not possible 3
4 This implies: Density Functional Theory The energy of the ground state of a manyelectron system : E 0 ({R I }) = Min Φ <Φ H e Φ> Hohenberg and Kohn (1964): The functional n(r) = n[φ] = <Φ δ(r r i ) Φ> can be inverted, i.e., Φ(r 1, r 2,..., r N, ) = Φ[n(r)]. E 0 ({R I }) = Min n(r) E {R} [n] i Comparison of Wave-Function and Density-Functional theory 4
5 Comparison of Wave-Function and Density-Functional theory Summary of Hohenberg-Kohn Density-Functional Theory (DFT) There is a one-to-one correspondence between the ground-state wave function and the many-body Hamiltonian [or the nuclear potential, v nuc (r)]. -- The many-body Hamiltonian determines everything. -- There is a one-to-one correspondence between the ground-state electron-density and the groundstate wave function. 5
6 Summary of Hohenberg-Kohn Density-Functional Theory (DFT) There is a one-to-one correspondence between the ground-state wave function and the many-body Hamiltonian [or the nuclear potential, v nuc (r)]. -- The many-body Hamiltonian determines everything. -- There is a one-to-one correspondence between the ground-state electron-density and the groundstate wave function. Kohn and Sham (1965): with And T s [n] the functional of the kinetic energy of noninteracting electrons. E xc [n] contains all the unknowns. At fixed electron number N the variational principle gives or = Kohn-Sham equation 6
7 Kohn and Sham (1965): Because T s [n] is the functional of non-interacting particles we effectively restrict the allowed densities to those that can be written as This implies: i i i v eff (r) depends on the density that we are seeking. i Kohn-Sham equation k k k The Kohn-Sham Ansatz -- Kohn-Sham (1965) Replace the original manybody problem with an independent electron problem that can be solved! -- Only the ground state density and the ground state energy are required to be the same as in the original many-body problem. -- Maybe the exact E xc [n] functional cannot be written as a closed mathematical expression. Maybe there is a detour similar to that taken for T s [n]? The challenge is to find useful, approximate xc functionals. 7
8 T s, E Hartree, and E xc are all universal functionals in n(r), i.e., they are independent of the special system studied. (general theory: see the work by Levy and Lieb) neglecting is the localdensity approximation n more about xc functionals: later this week The Exchange-Correlation Hole Comparison of Hartree, Hartree-Fock, and density-functional theories for jellium For non-jellium systems and the LDA (or the GGA) the shape of n xc (r,r ) is incorrect. However, only its spherical average enters: n xc (r, r ) E xc [n] = more about xc functionals comes later this week 8
9 The Exchange Hole in Ne Atom O. Gunnarsson, M. Jonson, and B. I. Lundqvist, Phys. Rev. B 20, 3136 (1979) exact exact LDA position of nucleus position of electron LDA LDA Exchange hole n x (r,r ) Spherical average around the electron Exchange-Correlation Hole in Silicon R. Q. Hood, M. Y. Chou, A. J. Williamson, G. Rajagopal, and R. J. Needs, PRB 57, 8972 (1998) The spherically averaged exchange-correlation hole in variational Monte Carlo (VMC) and DFT-LDA with (a) one electron fixed at the bond center, (b) one electron fixed at the tetrahedral interstitial site, and (c) plots (a) and (b) superimposed with the same scale. 9
10 Solving the Kohn- Sham Equations The self-consistent field method Structure, types of atoms Guess for input Solve KS Eqs. New density Self-consistent? Output: Total energy, force,... Eigenvalues update the geometry mixing total energy, forces, etc. (L)APW k-point summation The Total Energy From DFT a classical term V BO M <Λ 0 T Ion Λ 0 > a 0 zero point energy cohesive energy lattice parameter a 0 +Δa The total energy per atom without zero point vibrations as a function of the interatomic distance. The measured interatomic distance is the average over the positions of vibrating atoms. 10
11 Most-Cited Papers in APS Journals 11 papers published in APS journals since 1893 with >1000 citations in APS journals (~5 times as many references in all science journals) From Physics Today, June, years before DFT invention 34 years after DFT invention Professor Nixwi 11
12 From: Walter Kohn - Personal Stories and Anecdotes Told by Friends and Collaborators. M. Scheffler, P. Weinberger (Eds.), Springer-Verlag, Berlin Heidelberg New York 2003, 309 p. ISBN Challenges for the Near Future Quantum-Mechanics Based Technology Create new materials and systems by design, e.g. quantum dots, quantum wires, nano-catalysis, etc. For nanotechnology to become affordable, nanostructures will have to build themselves; normal manufacturing methods will be useless self-organization and assembly. Make progress in understanding biological systems starting from the fundamental equations of quantum mechanics. Bridging the time and length scales 12
13 length (m) lle cha electronic regime nge Density Functional Theory mesoscopic regime Statistical Mechanics or Thermodynamics macroscopic regime time (s) The first (convincing) DFT calculations: Stability of crystals and crystal phase transitions Total energy (Ryd./atom) β-tin silicon diamond Volume M. T. Yin and M. L. Cohen PRB 26 (1982) see also: V.L. Moruzzi, J.F. Janak, and A. R. Williams Calculated Electronic Properties of Metals Pergamon Press (1978) 13
14 Electron Density of Si Electron density difference from sum of atoms Exp LDA GGA experiment DFT-LDA DFT-GGA J. M. Zuo, P. Blaha, and K. Schwarz, J. Phys. Cond. Mat. 9, 7541 (1997) Ab initio melting curve of Fe as function of pressure Depth (km) ,700 2,890 lower mantle transition region outer core (liquid) 5,150 inner core 6, Pressure (GPa) Temperature (1000 K) Pressure (GPa) D. Alfe, M. J. Gillan, and G. D Price NATURE 401 (1999) 14
15 Bio-Physics: DFT vs Force Fields DFT-PBE AMBER π α 3 10 J. Ireta et al. CHARMM27 more: later this week both force fields predict the α- helix to be the most stable conformation only AMBER reproduces all the helical minima What About the Kohn-Sham Eigenvalues? The only quantities that are supposed to be correct in the Kohn-Sham approach are density, energy, forces, What about the individual φ i (r) and є i? The Kohn-Sham φ i (r) and є i give an approximate description of quasi-particles, a (good) starting point for many-body calculations. 15
16 What About the Kohn-Sham Eigenvalues? The ionization energy is: (Well defined for the highest occupied state. Otherwise, this only holds approximately.) Here we assume that the positions of the atoms don t change upon ionization, or that they change with some delay (Franck-Condon principle). Using the mean-value theorem of integral algebra gives: This is called the Slater-Janak transition state. It can be viewed as the DFT analog of Koopmans theorem. Silicon Kohn-Sham Electron Bands Kohn-Sham band gap: Δ = є LB - є VB of the N-particle system The measured (optical) band gap is something else: removal addition conduction band (empty states) } the KS gap Δ L Γ X W valence band (filled states) E gap = Modern calculations of E gap employ the GW approach, starting from KS energies and wave functions. more comes later this week 16
17 Summary -- Interacting electrons determine the properties and function of real materials and bio molecules. -- Approximate xc functionals have been very successful, but for highly correlated situations there are problems. Exciting arenas for theoretical work: -- Thermodynamic phase transitions, e.g. melting. -- Surfaces, interfaces, defects, nanostructures in realistic environments -- modeling kinetics, e.g. for catalysis or crystal growth (self-assembly and self-organization) -- Molecules and clusters in solvents, electrochemistry, fuel cells, external fields, transport. -- Biological problems The challenges: -- Find practical ways to correct the xc approximation. -- Develop methods for bridging the length and time scales. Three Basic Concepts for Modern Electronic Structure Calculations Plane waves The simplicity and speed of Fast Fourier Transforms Requires smooth pseudopotentials Localized orbitals The intuitive appeal of atomic-like states; good scaling with system size. Augmented methods Best of both worlds also most demanding. Most general form: (L)APW Be aware of the limitations of present xc functionals. Use codes properly and carefully. ENJOY THE WORKSHOP!!! 17
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