Tue.27 Nov.Wed.28 Nov.Thu.29 Nov. 8h30 - 9h45 Topical lecture Planets / High Pressure Topical lecture Atmospheric physics Methods: Ab-initio Molecular Dynamics 9h h15Coffee break 10h15 -11h30 Methods Plane waves, cut-off, k-points, pseudopotentials, DFT, etc SeminarsComputer Lab 3 11h30 – 13h30Lunch time 13h30 – 16h00Computer Lab 1Computer Lab 2Computer Lab 4 The 6th School on Simulation and Modeling Physics "Ab-initio methods and their applications" Hanoi, November 2007

Ab-initio molecular dynamics in atmospheric science Sandro Scandolo The Abdus Salam International Center for Theoretical Physics Trieste, Italy 6 th SMP, Hanoi, Nov 27-29, 2007

Two case studies: Electron attachment at the surface of ice (the chemistry of the ozone hole) Infrared absorption by small water clusters (understanding the greenhouse effect)

Two case studies: Electron attachment at the surface of ice (the chemistry of the ozone hole) Infrared absorption by small water clusters (understanding the greenhouse effect)

Stratospheric clouds in polar regions consists of ice micro/nanoparticles OZONE (O 3 ) OXYGEN (O 2 ) GOOD in stratosphere BAD in troposphere GOOD in troposphere BAD in stratosphere Motivations: stratospheric chemistry and ozone hole

CFC’s break down at the surface of ice microparticles and produce active Chlorine Motivations: stratospheric chemistry and ozone hole What causes the break down of CFCs at the surface of ice? (1) Sunlight (UV) radiation? (2) Excess electrons produced by cosmic rays?

Photolysis by UV photons Dissociation cross-section ~ cm 2 CF 2 Cl 2 +hv  CF 2 Cl+Cl Dissociative Electron Attachment by free e- Dissociation cross-section ~ cm 2 CF 2 Cl 2 +e -  CF 2 Cl+Cl - Dissociative Electron Attachment by trapped e- Dissociation cross-section ~ cm 2 CF 2 Cl 2 +e-(H 2 O) n  CF 2 Cl+(H 2 O) n +Cl - (Lu and Sanche, PRL 2001) Excess electrons get trapped in ice thin films (few monolayers) on Cu (M. Wolf et al, 2003, and to be published) Where do electrons prefer to stay after attachment to ice surfaces? How are chemical reactions at ice surfaces affected by excess electrons? Solvated? Pre-solvated? Cl - CF 2 Cl 2 Motivations: stratospheric chemistry and ozone hole

Surface or Bulk solvated state? Liquid water: The excess electron is completely solvated in liquid bulk water (Hart&Boag, JACS 1962) For small clusters the surface state is stabilized by a rearrangement of the molecular dipoles (Kim et al. JCP 2005) Water clusters Localization depends on the cluster size and structure ( Verlet et al. Science 2005, Paik et al. Science 2004) Motivations: excess electrons on ice Ice? A solvated state similar to that found in liquid water is the likely final state of an excess electron, but reaching this state is likely to take a very long time (microseconds)

Energy gap Filled states Empty states Vacuum level position energy Level alignment at insulating surfaces

Energy gap Filled states Empty states Vacuum level position energy Level alignment at insulating surfaces Negative electron affinity

Convergence with vacuum thickness

Surface states in polyethylene M.C. Righi et al., Phys. Rev. Lett. 87, (2001)

Ab-initio Molecular Dynamics 32 H 2 O molecules in Ih structure and complete proton disorder BLYP exchange-correlation functional and Martins-Troullier pseudopotentials Periodic boundary conditions Vacuum ~20 A Both neutral and charged (with excess electron) cases are considered Positive compensating background Self-interaction correction System evolved at T~150K Where do excess e - prefer to stay? Do they self-trap as in liquid water?

Self-Interaction problem w/o SIC with SIC The excess interacts with its own electrostatic potential in the vacuum region, producing a weird, unphysical charge density With standard (GGA) approximations to V xc, the charge density of the excess electron localizes in the vacuum region between ice slabs, however its charge distribution is unphysical ! Definition of V xc

Self-interaction correction for unpaired electrons (holes) Solution: Constrained Local Spin Density –DFT (d’Avezac et al, PRB 2005): “Paired” electron wave functions are forced to be equal for up and down spins for all i paired states Excess electron density is then given by an eigenfunction independent quantity: WHEN DOES IT WORK? When the paired eigenvalues of system from a LSD calculation are not so different When there is ONLY one charge in excess (e- or hole) The standard self-interaction correction turns the Hamiltonian H e into an eigenfunction-dependent operator

Self-interaction correction: single water molecule Charge density of excess electron with SIC H 2 O + e - LDA H 2 O LDA Blue: high red: low H 2 O + e - With SIC Electron affinity: +1.4 eV-0.1 eV Exp: ~0 eV

w/o SIC with SIC Self-interaction corrected excess electron on ice

Excess electron at the surface of ice Ih charge density of excess electron Excess electron localizes at the surface F. Baletto, C. Cavazzoni, S. Scandolo, PRL 95, (2005)

Neutral surface evolved for 1.6 ps Surface with excess electron evolved for 2.4 ps Additional dangling OH

Excess electron localized at the surface Additional dangling OH lowers work function by about 1.8 eV Formation of a subsurface cavity with repulsive character (the surface of the cavity is oxygen-rich) Work in progress: Add CFCs and check if dissociation is spontaneous in the presence of e-

Two case studies: Electron attachment at the surface of ice (the chemistry of the ozone hole) Infrared absorption by small water clusters (understanding the greenhouse effect)

(a) Dimer (b) Tetramer (c) Hexamer-ring (d) Hexamer-book (at 220 K) Binding energy (eV/molecule) Tetramer Hexamer Dimer MP2Ourssystem

Water vapor absorption is completely different from bulk ice/water Water vapor

Ab-initio molecular dynamics at 200 K

Absorption coefficient calculated from the MD trajectory Total dipole Strong temperature dependence!

Absorption coefficient for dimer at atmospheric conditions (220 K) Water vapor absorption Water dimers are only marginally responsible for water vapor absorption M.-S. Lee et al, Phys Rev. Lett, submitted

Thanks to Francesca Baletto (now at King’s College London) Mal-Soon Lee (ICTP) Carlo Cavazzoni (CINECA, Bologna) Diep Quang Vinh (now at Purdue, USA)