The 6th School on Simulation and Modeling Physics "Ab-initio methods and their applications"

  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

Convergence with vacuum thickness

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

w/o SIC

with SIC

Self-interaction corrected excess electron on ice

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

Water vapor absorption is completely different from bulk ice/water

1200400 800

Water vapor

Ab-initio molecular dynamics at 200 K

Absorption coefficientcalculated from the MDtrajectory

Total dipole

Strong temperaturedependence!

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

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)