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Tue.27 Nov. Wed.28 Nov. Thu.29 Nov.
8h30 - 9h45Topical lecture
Planets / High PressureTopical lecture
Atmospheric physics
Methods: Ab-initio Molecular
Dynamics
9h45 - 10h15 Coffee break
10h15 -11h30Methods
Plane waves, cut-off, k-points, pseudopotentials, DFT, etc
Seminars Computer Lab 3
11h30 – 13h30 Lunch time
13h30 – 16h00 Computer Lab 1 Computer Lab 2 Computer Lab 4
The 6th School on Simulation and Modeling Physics"Ab-initio methods and their applications" Hanoi, 27 - 29 November 2007
Ab-initio molecular dynamics in atmospheric science
Sandro Scandolo
The Abdus Salam International Center for Theoretical Physics Trieste, Italy www.ictp.it
6th 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 (O3)
OXYGEN (O2)
GOOD in stratosphere
BAD in troposphereGOOD 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 ~ 10-20 cm2
CF2Cl2+hvCF2Cl+Cl
Dissociative Electron Attachment by free e-
Dissociation cross-section ~ 10-16 cm2
CF2Cl2+e-CF2Cl+Cl-
Dissociative Electron Attachment by trapped e-
Dissociation cross-section ~ 10-14 cm2
CF2Cl2+e-(H2O)nCF2Cl+(H2O)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-
CF2Cl2
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 statesVacuum level
position
en
erg
yLevel alignment at insulating surfaces
Energy gap
Filled states
Empty states
Vacuum level
position
en
erg
yLevel alignment at insulating surfaces
Negative electron affinity
Surface states in polyethyleneSurface states in polyethylene
M.C. Righi et al., Phys. Rev. Lett. 87, 076802 (2001)
Ab-initio Molecular Dynamics
32 H2O 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 Vxc , the charge density of the excess electron localizes in the vacuum region between ice slabs, however its charge distribution is unphysical !
))(,(''
)'(2
rrVrdrr
r
rr
eeixc
i
e
j ji
Definition of Vxc
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 differentWhen there is ONLY one charge in excess (e- or hole)
))()(,(''
)'()'( 222
rrrVrdrr
rr
rr
eNexc
Ne
j j
The standard self-interaction correction
turns the Hamiltonian He into an eigenfunction-dependent operator
N
1N
2N
ii
2
N
Self-interaction correction: single water molecule
Charge density of excess electron with
SIC
H2O + e-
LDA
H2O
LDA
Blue: high red: low
H2O + e-
With SIC
Electron affinity:
+1.4 eV -0.1 eV Exp: ~0 eV
Excess electron at the surface of ice Ih
charge density ofexcess electron
Excess electron localizes at the surface
F. Baletto, C. Cavazzoni, S. Scandolo, PRL 95, 176801 (2005)
Neutral surface evolved for 1.6 ps Surface with excess electron evolved for 2.4 ps
Additional dangling OH
Excess electron localizedat the surface
Additional dangling OH lowerswork 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)
0.2640.27Tetramer
0.3070.30Hexamer
0.0950.09Dimer
MP2Ourssystem
Absorption coefficientfor dimer at atmosphericconditions (220 K)
Water vapor absorption
Water dimers are onlymarginally responsible for water vapor absorption
M.-S. Lee et al, Phys Rev. Lett, submitted
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