XIII International Conference on the applications of DFT in Chemistry and Physics Lyon 2nd September 2009 GW renormalization of DFT molecular electronic levels at the vicinity of a surface: The image charge effect Juan María García Lastra Kristian Sommer Thygesen Ángel Rubio Outline 1. Introduction 2. Motivation 3. Our work 4. A simple model to explain the results 5. Outlook Image charge 1.Introduction Metal q2 Vimg ( z ) 4( z z0 ) z q -q Semiconductor C60 on Ag(111) q2 ( 1) Vimg ( z ) r 4( z z0 ) ( r 1) Is it possible to reproduce this effect within DFT? R. Hesper, L.H. Tjeng and G.A. Sawatzky, Europhys. Lett. 40, 177 (1997) Some definitions and equivalences in DFT 1.Introduction Ionization Potential (IP) Electron affinity (EA) X IP X e X EA X e D IP EA Gap (D) DFT Vacuum LUMO LUMO Exact Vxc IP HOMO EA LUMO D LUMO HOMO C HOMO HOMO C is the derivative discontinuity J.P. Perdew and M. Levy Phys. Rev. Lett. 51, 1884 (1983) DSCF 1.Introduction Alternative : DSCF LUMO IP HOMO EA D=IP-EA + -2 Problem: EXTENDED SYSTEMS 1.Introduction Many Body Perturbation Theory The combination of a particle and its influence on the local environment Propagators S Self-energy R.D. Mattuck, A guide to Feynman Diagrams in the Many-Body Problem 1.Introduction GW approximation , DFT i Good enough DFT i L. Hedin, Phys. Rev. 139, A796 (1965) G0 i Initial guess B. I. Lundqvist, Phys. Kondens. Mater. 6, 193 (1967) P i EGS G i G G 0 , S G G 0 G 0 SG S i W i , QP i QP i F. Aryasetiawan and O. Gunnarsson, Rep. Prog. Phys. 61, 237 (1998) G. Onida, L. Reining and A. Rubio, Rev.Mod.Phys. 74, 601 (2002) DFT vs. GW 1. Introduction DFT + local xc-functionals underestimate HOMO-LUMO gaps Hartree-Fock is good for small molecules (SI-free), but overestimates the gap for extended systems GW includes screening in the exchange and this solves the gap problem. Schilfgaarde, Kotani, and Faleev, PRL 96, 226402 (2006) Hartree-Fock exchange Screening correction 2.Motivation Theoretical interest 2.Motivation D. G. de Oteyza, J.M. García-Lastra et al., Adv. Func. Mater., accepted STM 2.Motivation Molecules and layers on surfaces DIP and F16CuPc on Cu(111) D. G. de Oteyza, J.M. García-Lastra et al., Adv. Func. Mater., in press Aromatic molecules on Cu(110) N. Atodiresei, V. Caciuc et al., PRL 102, 136809 (2009) Conductance at molecular junctions 2.Motivation Amine-Gold Linked Single-Molecule Circuits SY Quek et al., Nano Lett 7, 3477 (2007) Image Charge by dielectrics S D SiO2 K. Kaasbjerg and K. Flensberget, Nano Lett 8, 3809 (2008) 2.Motivation Conductance at molecular junctions SY Quek et al., Nano Lett 7, 3477 (2007) 3.Our work First-principles GW calculations: Physisorbed benzene 9 Å >Z>4 Å DFT calculations performed with PWSCF code (#) G0W0 calculations performed with the Yambo code(*). Yambo: G0W0 LDA, Plane wave basis, norm-conserving pseusopotentials, plasmon pole approximation. (#) S. Baroni et al. (2009), QUANTUM ESPRESSO package, www.quantumespresso.org/ (*) A. Marini, C. Hogan, M. Grüning, D. Varsano, Comp. Phys. Comm. 180, 1392 (2009). See also: J. B. Neaton et al. Phys. Rev. Lett. 97, 216405 (2006) Benzene Molecule 3.Our work •Previously obtained by Neaton et al. •LDA underestimates the gap by a factor of 2 (mainly due to Selfinteraction) 5.2 eV 10.5 eV •GW HOMO-LUMO gap agrees with experiment (IP-EA) •LUMO predicted to be above the vacuum level in GW, in agreement with experiment Experiment: IP = 9.25 eV EA = -1.15 eV L. Klasinc et al., Pure Appl. Chem. 55, 289 (1983) B.T. Hill, J. Chem. Soc. Perkin Trans. II 1027 (1998) D = 10.4 eV Substrates 3.Our work Insulator and semiconductor NaCl(001) MgO(001) 8.9 eV 7.7 eV •Same structure (fcc) CaO(001) BaO(001) 6.3 eV 4.0 eV BaO(111) •Varying the gap •Varying the surface Metallic surface! Substrates 3.Our work Metals Pt(111) Rh(111) Ti(001) Al(111) Li(001) sd sd sd sp s •Different DOS at Fermi Level •Similar interatomic distances •Except Li: Electrons outer of the surface Substrates 3.Our work Semimetallic •Benzene on Graphite(0001) •Previously studied by Neaton, Hybertsen and Louie, PRL 97, 216405 (2006) •Neaton et al. z = 3.25 Å •Our work 4 Å < z < 9 Å 3.Our work GW and LDA benzene HOMO-LUMO gaps 4.5 Å J.M.G-L, A. R. and K.S.T., submitted LDA gaps are independent of substrate and distance Same result with other functionals (GGA, hybrid or exact exchange) GW gaps show large variation across different surfaces GW gap sensitive to atomistic details, e.g. surface plane (BaO) 3.Our work Classical image charge model Electrostatic energy of point charge above a polarizable medium: q2 ( 1) Vimg ( z ) r 4( z z0 ) ( r 1) Classical model describes the physics of the gap reduction qualitatively. Best-fit values for and z0: Fitted for the gap: Different values if HOMO or LUMO are fitted independently Dynamic interaction between benzene orbitals and surfaces: Bulk Dielectric Constant is not a good descriptor 3.Our work Variation of HOMO and LUMO levels Vacuum Vacuum GW: Symmetric effect on HOMO and LUMO. Exceptions Li and BaO(111) LDA: HOMO level agrees better with GW than does LUMO Very good agreement between LDA and GW for HOMO at metallic surfaces (error cancellation in LDA between self-interaction and image charge) 3.Our work General trends in level shifts Insulator and semiconductor Gap reduction increases with decreasing substrate band gap General trends in level shifts 3. Our work Metals Gap reduction increases with increasing substrate DOS at EF Li and BaO(111) deviate from general trend! 4. A simple model to explain the results GW S to second order in V Renormalization of single electronic level, interactions with substrate electrons: Hartree-Fock exchange , by non-local Screening correction We truncate the expansion in the second order term 4. A simple model to explain the results Semiconductors L Effective interaction strength L L L Substrate joint density of states weighted by particle-hole transitions Metals A simple model to explain the results L L proportional to JDOS Slope of JDOS at =0 proportional to DOS at EF The correction increases if DOS at EF increases 5.Outlook •DFT is not able to reproduce image charge effect •GW includes dynamic correlation (polarization) and solves the problem •Classic image potential describes the effect phenomenologically •However microscopic description is required •Renormalization of the gap in molecules follows the band gap in semiconductors •Renormalization of the gap in molecules follows the DOS at Fermi level in metals •It is possible to understand the results truncating at second order the self energy. A simple model to explain the results Metals