Excited States and GW/BSE · 2011. 7. 22. · Excited States and GW/BSE Patrick Rinke...

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Excited States and GW /BSE

Patrick Rinke

Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin - Germany

Hands-on Tutorial on Ab Initio Molecular Simulations: Toward aFirst-Principles Understanding of Materials Properties and Functions

Patrick Rinke (FHI) Excited States Berlin 2011 1 / 54

Excited States in (Material) Science – Are Ubiquitous

Theoretical SpectroscopyExperiment/Spectroscopy

appropriate?

accurate?

Material Properties + Applications

appropriate?

accurate?

Patrick Rinke (FHI) Excited States Berlin 2011 2 / 54

Excited States in (Material) Science – Are Ubiquitous

Theoretical SpectroscopyExperiment/Spectroscopy

appropriate?

accurate?

Material Properties + Applications

appropriate?

accurate?

photoemission

optical absorption DFT+ +BSEG W0 0

DFT+G W0 0

Patrick Rinke (FHI) Excited States Berlin 2011 2 / 54

Band structures: photo-electron spectroscopy

- -

-

-

+

Photoemission

hn

GW

- -

-

Inverse Photoemission

-

-

hn

-

GW

- -

-+

Absorption

-

hn

BSETDDFT

Patrick Rinke (FHI) Excited States Berlin 2011 3 / 54

Photo-Electron Excitation Energies

Photoemission electron removal

|N〉

removal energy

E(N)

EF

Evac

- -

-

hn

-

N-electrons

Photoemission

|N〉 : N -particle ground state

E(N, s) : total energy in state s

Patrick Rinke (FHI) Excited States Berlin 2011 4 / 54

Photo-Electron Excitation Energies

Photoemission electron removal

ψ(r)|N〉

removal energy

E(N)

EF

es

Evac

- -

-

ef-

+

(N-1)-electrons

Ekin

Photoemission

hn

|N〉 : N -particle ground state

E(N, s) : total energy in state s

Patrick Rinke (FHI) Excited States Berlin 2011 4 / 54

Photo-Electron Excitation Energies

Photoemission electron removal

〈N − 1, s|ψ(r)|N〉

removal energy

E(N)− E(N − 1, s)

EF

es

Evac

- -

-

ef-

+

(N-1)-electrons

Ekin

Photoemission

hn

|N〉 : N -particle ground state|N − 1, s〉 : (N − 1)-particle excited state sE(N, s) : total energy in state s

Patrick Rinke (FHI) Excited States Berlin 2011 4 / 54

Photo-Electron Excitation Energies

Photoemission electron removal

ψs(r) = 〈N − 1, s|ψ(r)|N〉

removal energy

ǫs = E(N)− E(N − 1, s)

EF

es

Evac

- -

-

ef-

+

(N-1)-electrons

Ekin

Photoemission

hn

|N〉 : N -particle ground state|N − 1, s〉 : (N − 1)-particle excited state sE(N, s) : total energy in state s

Patrick Rinke (FHI) Excited States Berlin 2011 4 / 54

Photo-Electron Excitation Energies

Photoemission electron removal

ψs(r) = 〈N − 1, s|ψ(r)|N〉

removal energy

ǫs = E(N)− E(N − 1, s)

Inverse Photoemission electron addition

ψs(r) = 〈N |ψ(r)|N + 1, s〉

addition energy

ǫs = E(N + 1, s)− E(N)

EF

es

Evac

- -

-

ef-

+

(N-1)-electrons

Ekin

Photoemission

hn

|N〉 : N -particle ground state|N − 1, s〉 : (N − 1)-particle excited state sE(N, s) : total energy in state s

Patrick Rinke (FHI) Excited States Berlin 2011 4 / 54

Single Particle Green’s Functions

Lehmann representation of single particle Green’s function G:

G(r, r′; ǫ) = limη→0+

s

ψs(r)ψ∗s(r

′)

ǫ− (ǫs + iη sgn(Ef − ǫs))

Excitation energies are poles of the Green’s function!

spectroscopically relevant quantity: spectral function A:

A(ǫ) = −1

π

dr limr′→r

ImG(r, r′; ǫ)

A( )e

es

h

e

Patrick Rinke (FHI) Excited States Berlin 2011 5 / 54

Single Particle Green’s Functions

Lehmann representation of single particle Green’s function G:

G(r, r′; ǫ) = limη→0+

s

ψs(r)ψ∗s(r

′)

ǫ− (ǫs + iη sgn(Ef − ǫs))

Excitation energies are poles of the Green’s function!

spectroscopically relevant quantity: spectral function A:

A(ǫ) = −1

π

dr limr′→r

ImG(r, r′; ǫ)

A( )e

es

h

e

Patrick Rinke (FHI) Excited States Berlin 2011 5 / 54

Experimental: angle-resolved photoemission spectroscopy

ARPES

=+1qo

Binding Energy (eV)8 6 4 2 0

Masaki Kobayashi, PhD dissertation

Patrick Rinke (FHI) Excited States Berlin 2011 6 / 54

Experimental: angle-resolved photoemission spectroscopy

ARPES

=+1qo

Binding Energy (eV)8 6 4 2 0

Masaki Kobayashi, PhD dissertation

Patrick Rinke (FHI) Excited States Berlin 2011 6 / 54

ARPES vs GW : the quasiparticle concept

Quasiparticle:

single-particle like excitation

A ( )k

e spectralfunction

e

Patrick Rinke (FHI) Excited States Berlin 2011 7 / 54

ARPES vs GW : the quasiparticle concept

Quasiparticle:

single-particle like excitation

Ak(ǫ) = ImGk(ǫ) ≈Zk

ǫ− (ǫk + iΓk)

ǫk : excitation energyΓk : lifetimeZk : renormalisation

A ( )k

e quasiparticle-peak

ek

qp

Gk

e

Patrick Rinke (FHI) Excited States Berlin 2011 7 / 54

ARPES vs GW : the quasiparticle concept

Quasiparticle:

single-particle like excitation

Ak(ǫ) = ImGk(ǫ) ≈Zk

ǫ− (ǫk + iΓk)

ǫk : excitation energyΓk : lifetimeZk : renormalisation

A ( )k

e quasiparticle-peak

ek

qp

Gk

e

A ( )k e quasiparticle-

peak

ek

qp

Gkes

e

Patrick Rinke (FHI) Excited States Berlin 2011 7 / 54

Green’s function and screening

-

-

-

-

-

---

--

Patrick Rinke (FHI) Excited States Berlin 2011 8 / 54

Green’s function and screening

-

-

-

-

-

-

--- -

-

-

--

+

ejected

system polarizes

tt-

-- -

- -

Patrick Rinke (FHI) Excited States Berlin 2011 8 / 54

Green’s function and screening

-

-

-

-

-

-

---

-

-

-

-

-+- -

-

-

--

+

ejected

system polarizes

tt tt

polarization is

time-dependent

hole is screened dynamically

-

--

--

- -

- -

Patrick Rinke (FHI) Excited States Berlin 2011 8 / 54

Green’s function and screening

-

-

-

-

-

-

---

-

-

-

-

-+- -

-

-

--

+

ejected

system polarizes

tt tt

polarization is

time-dependent

hole is screened dynamically

-

--

--

- -

- -

quasiparticle

Patrick Rinke (FHI) Excited States Berlin 2011 8 / 54

GW Approximation - Screened Electrons

-

-

-

-

-

-+

-

--

G: propagator

W

GS=GW :

W: screened

Coulomb

Self-Energy

ΣGW (r, r′, ω) = −i

dωeiωηG(r, r′, ω + ω′)W (r, r′, ω′)

Patrick Rinke (FHI) Excited States Berlin 2011 9 / 54

Green’s function is solution to Hedin’s equations

Hedin’s equations - exact L. Hedin, Phys. Rev. 139, A796 (1965)

notation: 1 = (r1, σ1, t1)

P (1, 2) = −i

G(2, 3)G(4, 2+)Γ(3, 4, 1)d(3, 4)

W (1, 2) = v(1, 2) +

v(1, 3)P (3, 4)W (4, 2)d(3, 4)

Σ(1, 2) = i

G(1, 4)W (1+, 3)Γ(4, 2, 3)d(3, 4)

Γ(1, 2, 3) = δ(1, 2)δ(1, 3) +

∫δΣ(1, 2)

δG(4, 5)G(4, 6)G(7, 5)Γ(6, 7, 3)d(4, 5, 6, 7)

Dyson’s equations

G−1(1, 2) = G−10 (1, 2)− Σ(1, 2)

links non-interacting (G0) with interacting (G) system

Patrick Rinke (FHI) Excited States Berlin 2011 10 / 54

Green’s function is solution to Hedin’s equations

Hedin’s equations - exact L. Hedin, Phys. Rev. 139, A796 (1965)

notation: 1 = (r1, σ1, t1)

P (1, 2) = −i

G(2, 3)G(4, 2+)Γ(3, 4, 1)d(3, 4)

W (1, 2) = v(1, 2) +

v(1, 3)P (3, 4)W (4, 2)d(3, 4)

Σ(1, 2) = i

G(1, 4)W (1+, 3)Γ(4, 2, 3)d(3, 4)

Γ(1, 2, 3) = δ(1, 2)δ(1, 3) +

∫δΣ(1, 2)

δG(4, 5)G(4, 6)G(7, 5)Γ(6, 7, 3)d(4, 5, 6, 7)

Dyson’s equations

G−1(1, 2) = G−10 (1, 2)− Σ(1, 2)

links non-interacting (G0) with interacting (G) system

exact : yes

tractable: no

But, do not despair!

Patrick Rinke (FHI) Excited States Berlin 2011 10 / 54

Green’s function is solution to Hedin’s equations

Hedin’s GW equations L. Hedin, Phys. Rev. 139, A796 (1965)

notation: 1 = (r1, σ1, t1)

Γ(1, 2, 3) = δ(1, 2)δ(1, 3)

P (1, 2) = −i

G(2, 3)G(4, 2+)Γ(3, 4, 1)d(3, 4)

= −iG(1, 2+)G(2, 1)

W (1, 2) = v(1, 2) +

v(1, 3)P (3, 4)W (4, 2)d(3, 4)

Σ(1, 2) = i

G(1, 4)W (1+, 3)Γ(4, 2, 3)d(3, 4)

= iG(1, 2)W (1+ , 2)

Patrick Rinke (FHI) Excited States Berlin 2011 11 / 54

G0W0 Approximation in Practise

1 start from Kohn-Sham calculation for ǫKSs and φKS

s (r)

2 KS Green’s function:

G0(r, r′; ǫ) = lim

η→0+

s

φKSs (r)φKS∗

s (r′)

ǫ− (ǫKSs + iη sgn(Ef − ǫKS

s ))

3 Polarisability:

χ0(r, r′; ǫ) = −

i

dǫ′G0(r, r′; ǫ′ − ǫ)G0(r

′, r; ǫ′)

4 Dielectric function:

ε(r, r′, ǫ) = δ(r − r′)−

dr′′v(r− r′′)χ0(r

′′, r′; ǫ)

Patrick Rinke (FHI) Excited States Berlin 2011 12 / 54

G0W0 Approximation in Practise

5 Screened interaction:

W0(r, r′, ǫ) =

dr′′ε−1(r, r′′; ǫ)v(r′′ − r′)

6 G0W0 self-energy:

Σ(r, r′; ǫ) =i

dǫ′eiǫ′δG0(r, r

′; ǫ+ ǫ′)W0(r, r′; ǫ′)

7 Quasiparticle equation:

h0(r)ψs(r) +

dr′Σ(r, r′; ǫqps )ψs(r′) = ǫqps ψs(r)

solve for quasiparticle energies ǫqps and wavefunctions ψs(r)

perturbation theory: ψs(r) = φKSs (r)

⇒ ǫqps = ǫKSs + 〈s|Σ(ǫqps )|s〉 − 〈s|vxc|s〉

Patrick Rinke (FHI) Excited States Berlin 2011 13 / 54

GW Approximation - Screened Electrons

-

-

-

-

-

-+

-

--

G: propagator

W

GS=GW :

W: screened

Coulomb

Self-Energy: Σ = Σx + Σc

Σx = iGv: exact (Hartree-Fock) exchange

Σc = iG(W − v): correlation (screening due to other electrons)

Patrick Rinke (FHI) Excited States Berlin 2011 14 / 54

On the importance of screening – Silicon

ǫqpnk = ǫLDA

nk + 〈φnk|Σx +Σc(ǫqpnk)− vxc|φnk〉

exp: 1.17 eV

Hartree-Fock (exchange-only) gap much too large

Screening is very important in solids!

Patrick Rinke (FHI) Excited States Berlin 2011 15 / 54

ARPES – GW: wurtzite ZnO

H L A G K M G

Ener

gy r

elat

ive

to(e

V)

EF

0

-2

-4

-6

ZnO G W0 0

@OEPx(cLDA)

Yan, Rinke, Winkelnkemper, Qteish, Bimberg, Scheffler, Van de Walle,

Semicond. Sci. Technol. 26, 014037 (2011)

Patrick Rinke (FHI) Excited States Berlin 2011 16 / 54

wurtzite ZnO – convergence of G0W0

0 1000 2000 3000number of bands

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3ba

nd g

ap (

eV)

5 10 15 20 25 30 35band cutoff energy (Ha)

2.6

2.7

2.8

2.9

3.0

3.1

3.2

3.3ba

nd g

ap (

eV)

wz-ZnO

plane wave cutoff: 35 Ha

G0W

0@OEPx(cLDA)

see also related work by Shih, Xue, Zhang, Cohen and Louie, PRL 105, 146401 (2010)

Patrick Rinke (FHI) Excited States Berlin 2011 17 / 54

LDA and G0W0 – the band gap question

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0Experimental Band Gap (eV)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

The

oret

ical

Ban

d G

ap (

eV)

LDAG

0W

0@LDA

selected group:IV, III-V, IIa-VI, IIb-VIcompounds

courtesy of Ricardo Gomez-Abal

Patrick Rinke (FHI) Excited States Berlin 2011 18 / 54

DFT eigenvalues and excitation energies

Density-functional theory:

in exact DFT: ionization potential given byKohn-Sham eigenvalue of highest occupied state

IKS = −ǫN (N)

otherwise Janak’s theorem: (PRA 18, 7165 (1978)))

∂E

∂ns= ǫs

rearranging and making mid point approximation:

E(N + 1, s)−E(N) =

∫ 1

0

dn ǫs(n) ≈ ǫs(0.5)

Patrick Rinke (FHI) Excited States Berlin 2011 19 / 54

Ionisation Potential, Electron Affinity and (Band) Gaps

Could use total energy method to compute

ǫs = E(N ± 1, s)− E(N)

Ionisation potential: minimal energy to remove an electron

I = E(N − 1)− E(N)

Electron affinity: minimal energy to add an electron

A = E(N)− E(N + 1)

(Band) gap: Egap = I −A

Patrick Rinke (FHI) Excited States Berlin 2011 20 / 54

Ionisation Potential, Electron Affinity and Band Gap

He Be Mg Ca Sr Cu Ag Au Li Na K Rb Cs Zn-60.0

-50.0

-40.0

-30.0

-20.0

-10.0

0.0

∆I in

%

E(N-1)-E(N)-ε

N(N)

Ionisation potential in the LDA

Reference: NIST – Atomic reference data

Patrick Rinke (FHI) Excited States Berlin 2011 21 / 54

∆SCF versus eigenvalues for finite systems

∞≈≈

data courtesy of Max Pinheiro

Patrick Rinke (FHI) Excited States Berlin 2011 22 / 54

Band Gap of Semiconductors

Band gap: Egap = I −A = E(N + 1)− 2E(N) + E(N − 1)

For solids E(N + 1) and E(N − 1) cannot be reliable computed

In Kohn-Sham the highest occupied state is exact

Egap = ǫKSN+1(N + 1)− ǫKS

N (N)

= ǫKSN+1(N + 1)− ǫKS

N+1(N)︸ ︷︷ ︸

∆xc

+ ǫKSN+1(N)− ǫKS

N (N)︸ ︷︷ ︸

EKSgap

For solids: N ≫ 1 ⇒ ∆n(r) → 0 for N → N + 1

⇒ discontinuity in vxc upon changing the particle number

∆xc =

(δExc[n]

δn(r)

∣∣∣∣N+1

−δExc[n]

δn(r)

∣∣∣∣N

)

+O

(1

N

)

Patrick Rinke (FHI) Excited States Berlin 2011 23 / 54

Ionisation Potential, Electron Affinity and Band Gap

so what about ∆SCF for excitations?

∆SCF: ǫs = EDFT(N)− EDFT(N ± 1, s)

reasonably good for I and A of small finite systems

but:

most functionals suffer from delocalization or self-interaction error(Science 321, 792 (2008))

⇒ the more delocalized the state, the larger the error

only truely justified for differences of ground states ionisation potential, electron affinity excited states that are ground states of particular symmetry

difficult to find excited state wavefunction (density)

excited state density is not unique

separate calculation for every excitation needed not practical for large systems or solids

Patrick Rinke (FHI) Excited States Berlin 2011 24 / 54

Can we also describe localized d- or f -electrons?

-8 -6 -4 -2 0 2 4 6 8 10Energy (eV)

0

2

4

6

8

10

12D

ensi

ty o

f St

ates

XPS+BISXPS+XAS

Ce2O

3 LDA

DFT-LDA erroneously predicts metallic state

Exp: J. W. Allen, J. Magn. Magn. Mater. (1985), D. R. Mullins et al., Surf. Sci. (1998)

Patrick Rinke (FHI) Excited States Berlin 2011 25 / 54

Ce2O3 in LDA+U

-8 -6 -4 -2 0 2 4 6 8 10Energy (eV)

0

2

4

6

8

10

12D

ensi

ty o

f St

ates

XPS+BISXPS+XAS

Ce2O

3 LDA+U

f-d

p-d

LDA+U splits f -peaks and opens gap

Patrick Rinke (FHI) Excited States Berlin 2011 26 / 54

f -Electron Systems – Lanthanide Oxides

f -Electrons

localized

itinerant

localized and itinerant electrons

⇒ rich physics and chemistry

LDA/GGA often not adequate

often termed strongly correlated

Lanthanide Oxides

Ln2O3 (Ln=lanthanide series)

technologically importantmaterials e.g. catalysis

Patrick Rinke (FHI) Excited States Berlin 2011 27 / 54

f -Electron Systems – Lanthanide Oxides

f -Electrons

localized

itinerant

localized and itinerant electrons

⇒ rich physics and chemistry

LDA/GGA often not adequate

often termed strongly correlated

Lanthanide Oxides

Ln2O3 (Ln=lanthanide series)

technologically importantmaterials e.g. catalysis

How well will GW perform?

exact exchange essential for localized electrons

screening essential for itinerant electrons

answers from new LAPW-based GW code

Patrick Rinke (FHI) Excited States Berlin 2011 27 / 54

Ce2O3 in G 0W 0@LDA+U

-8 -6 -4 -2 0 2 4 6 8 10Energy (eV)

0

2

4

6

8

10

12D

ensi

ty o

f St

ates

XPS+BISXPS+XAS

Ce2O

3 G0W

0@LDA+U

f-d

p-d

G 0W 0@LDA+U reproduces main peaks and gaps

H. Jiang, R. Gomez-Abal, P. Rinke and M. Scheffler, Phys. Rev. Lett. 102, 126403 (2009)

Patrick Rinke (FHI) Excited States Berlin 2011 28 / 54

GW for surfaces – CO on NaCl

gas phasechemisorbed/

physisorbed

change upon absorption: chemical bonding polarization of surface polarization of molecule

PRL 103, 056803 (2009), PRL 102, 046802 (2009), PRL 97 216405 (2006)

Patrick Rinke (FHI) Excited States Berlin 2011 29 / 54

GW for surfaces – CO on NaCl

gas phasechemisorbed/

physisorbed

-

+-

-

- +

image effect

change upon absorption: chemical bonding polarization of surface polarization of molecule

PRL 103, 056803 (2009), PRL 102, 046802 (2009), PRL 97 216405 (2006)

Patrick Rinke (FHI) Excited States Berlin 2011 29 / 54

GW for surfaces – CO on NaCl

Na ClOC

change upon absorption: chemical bonding polarization of surface polarization of molecule

HOMO-LUMO gap of CO:

gap/eV LDA G0W0@LDA

free CO : 6.9 15.1CO@NaCl: 7.4 13.1

surface polarization significant here

PRL 103, 056803 (2009), PRL 102, 046802 (2009), PRL 97 216405 (2006)

Patrick Rinke (FHI) Excited States Berlin 2011 29 / 54

GW for surfaces – CO on Ultrathin NaCl on Ge

CO

NaCl

Ge

Supported ultrathin films are novel nano-systems withunexpected features (PRL 99, 086101 (2007))

Additional electrons/holes on CO see two interfaces

Will the CO gap depend on NaCl thickness?

Patrick Rinke (FHI) Excited States Berlin 2011 30 / 54

GW for surfaces – CO on Ultrathin NaCl on Ge

12.512.612.712.8

G0W

0@LDA

2 3 4NaCl layers

7.47.57.67.77.8

DFT-LDA

5σ-2

π∗ spl

ittin

g (e

V)

3.03.13.23.33.4

2π∗

2 3 4NaCl layers

-2.1-2.0-1.9-1.8-1.7

G0W

0 cor

rect

ions

(eV

)Polarization at NaCl/Ge interface gives layer-dependent CO gap

⇒ molecular levels can be tuned by polarization engineering: interesting for molecular electronics and quantum transport

C. Freysoldt, P. Rinke, and M. Scheffler, PRL 103, 056803 (2009)

Patrick Rinke (FHI) Excited States Berlin 2011 31 / 54

Can we Eliminate Periodic Boundary Conditions in GW for

Finite Systems?

?

Patrick Rinke (FHI) Excited States Berlin 2011 32 / 54

FHI-aims – FHI ab initio molecular simulations package

Local atom-centered basis

ϕi[lm](r) =ui(r)

rYlm(Ω)

Standard DFT (LDA, GGA)

all-electron

periodic, cluster systems on equalfooting

favorable scaling (system size andCPUs)

Beyond DFT (only for finite systems)

Hartree-Fock, hybrid functionals,MP2, RPA

quasiparticle self-energies: GW ,MBPT2

V. Blum, R. Gehrke, F. Hanke, P. Havu, V. Havu, X. Ren, K. Reuter, and M. Scheffler,

Comp. Phys. Comm. 180, 2175 (2009).

Patrick Rinke (FHI) Excited States Berlin 2011 33 / 54

CO revisited – Comparison of FHI-aims and GWST

G0W0@LDA HOMO-LUMO gap of gas phase CO:

GWST† : 15.10 eVFHI-aims : 15.05 eVexperiment∗ : 15.80 eV

†GWST: GW space-time code (real-space/plane-waves)(Steinbeck, Godby et al. CPC 117 (1999), CPC 125 (2000))

∗Constants of Diatomic Molecules (1979), PRL 22, 1034 (1969)

Patrick Rinke (FHI) Excited States Berlin 2011 34 / 54

CO revisited – G0W0 with Different Starting Points

CO/eV ǫHOMO ǫLUMO

LDA -9.12 -2.25PBE -9.04 -2.00PBE0 -10.75 -0.75

G0W0@LDA -13.31 1.74G0W0@PBE -13.17 1.84G0W0@PBE0 -13.76 2.02

experiment∗ -14.00 1.80

PBE0 improves over LDA and PBE here

G0W0@PBE0 gives the best performance(X. Ren, P. Rinke, and M. Scheffler, Phys. Rev. B 80,

045402 (2009))

∗Constants of Diatomic Molecules (1979), PRL 22, 1034 (1969)

Patrick Rinke (FHI) Excited States Berlin 2011 35 / 54

GW – The Issue of Self-Consistency

Hedin’s GW equations:

G(1, 2) = G0(1, 2) notation: 1 = (r1, σ1, t1)

Γ(1, 2, 3) = δ(1, 2)δ(1, 3)

P (1, 2) = −iG(1, 2)G(2, 1+)

W (1, 2) = v(1, 2) +

v(1, 3)P (3, 4)W (4, 2)d(3, 4)

Σ(1, 2) = iG(1, 2)W (2, 1)

Dyson’s equation:

G−1 (1, 2) = G−10 (1, 2) − Σ (1, 2)

self-

consistency

self-consisten

cy

Patrick Rinke (FHI) Excited States Berlin 2011 36 / 54

CO – self-consistent GW

-20 -15 -10 -5 0 5Energy [eV]

Spec

tral

fun

ctio

n

G0W

0@LDA

sc-GW

-14 -13

Exp

.

Patrick Rinke (FHI) Excited States Berlin 2011 37 / 54

CO revisited – self-consistent GW

CO/eV ǫHOMO ǫLUMO

G0W0@LDA -13.31 1.74G0W0@PBE -13.17 1.84G0W0@PBE0 -13.76 2.02scGW -13.63 2.16

experiment∗ -14.00 1.80

scGW independent of starting point

G0W0@PBE0 outperforms scGW in this case

∗Constants of Diatomic Molecules (1979), PRL 22, 1034 (1969)

Patrick Rinke (FHI) Excited States Berlin 2011 38 / 54

Dependence on the starting point: N2

Galitskii-Migdal formula: Etot = −i

2

Tr [(ω + h0)G(ω)]dω

1 2 3 4 5 6 7Iteration

-109.8

-109.6

-109.4

-109.2T

otal

Ene

rgy

[Ha]

scGW@PBEscGW@HF

N2

-20 -18 -16 -14Energy [eV]

Spec

trum

G0W0@HFG0W0@PBEscGW@HFscGW@PBE

N2

At self-consistency no dependence on input Green’s function!

Patrick Rinke (FHI) Excited States Berlin 2011 39 / 54

scGW and ionization potentials

4 6 8 10 12 14 16Experimental IEs [eV]

4

6

8

10

12

14

16

The

oret

ical

IE

s [e

V]

4 6 8 10 12 14 16

4

6

8

10

12

14

16 LDA eigenvalueG

0W

0@LDA

Self-Consistent GWSelf-Consistent GW

0

deviation from exp.

LDA 40.0%G0W0@LDA 5.6%scGW 2.9%GW0@LDA 1.5%

set taken from Rostgaard, Jacobsen, and Thygesen, PRB 81, 085103, (2010)

Patrick Rinke (FHI) Excited States Berlin 2011 40 / 54

The scGW density

density from Green’s function: ρ(r) = −i∑

σ Gσσ(r, r, τ = 0+)

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Band structures: photo-electron spectroscopy

- -

-

-

+

Photoemission

hn

GW

- -

-

Inverse Photoemission

-

-

hn

-

GW

- -

-+

Absorption

-

hn

BSETDDFT

Patrick Rinke (FHI) Excited States Berlin 2011 42 / 54

Optical absorption – independent particles

Absorption spectrum = Im εM (ω) (macroscopic dielectric constant)

Eg+

-

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Optical absorption – independent particles

Absorption spectrum = Im εM (ω) (macroscopic dielectric constant)

Eg

Fermi’s Golden rule:

Im εM (ω) =16π

ω2

occ∑

v

unocc∑

c

|〈ψv|v|ψc〉|2 δ(ǫc − ǫv − ω)

v : velocity operator

Patrick Rinke (FHI) Excited States Berlin 2011 43 / 54

Optical absorption – response function

Dielectric constant:

Im εM (ω) = limq→0

1

ε−1

G=0,G′=0(q, ω)

Dielectric function:

ε−1(r, r′, ω) = δ(r− r′)−

dr′′v(r− r′′)χ(r′′, r′, ω)

Response function (in TDDFT):

χ = χ0 + χ0

[

v + fxc

]

χ

Response function (in RPA):

χ = χ0 + χ0

[

v +fxc

]

χ

Patrick Rinke (FHI) Excited States Berlin 2011 44 / 54

Optical absorption – response function

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0ω (eV)

0

10

20

30

40

50

60

Im ε

(ω)

Exp.TDLDARPA

absorption spectrum of silicon

from Sottile, Olevano, and Reining, PRL 91, 056402 (2003)

Patrick Rinke (FHI) Excited States Berlin 2011 45 / 54

Optical absorption – response function

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0ω (eV)

0

10

20

30

40

50

60

Im ε

(ω)

Exp."GW"TDLDARPA

absorption spectrum of silicon

from Sottile, Olevano, and Reining, PRL 91, 056402 (2003)

Patrick Rinke (FHI) Excited States Berlin 2011 45 / 54

Optical absorption – concept of excitons

Eg+

-

Patrick Rinke (FHI) Excited States Berlin 2011 46 / 54

Optical absorption – concept of excitons

Eg+

-exciton

electron-hole interaction lowers the energy

electron-hole pair (exciton) forms

Patrick Rinke (FHI) Excited States Berlin 2011 46 / 54

Optical absorption – concept of excitons

Eg

excitonic state

electron-hole interaction lowers the energy

electron-hole pair (exciton) forms

Patrick Rinke (FHI) Excited States Berlin 2011 46 / 54

Optical absorption – concept of excitons

+

-

+

-

G(1,2) G(1,2,1’,2’)

one particle

Green’s function

two particle

Green’s function

related quantity: two particle response function S(1, 2, 1′, 2′)

Patrick Rinke (FHI) Excited States Berlin 2011 47 / 54

Optical absorption – Bethe-Salpeter equation

Approximations for G(1, 2, 1′, 2′) (or S(1, 2, 1′, 2′))

GW approximation for Σ

approximateδΣ

δGby iW

take W to be static

⇒ effective eigenvalue problem in electron-hole basis(like Casida’s equation)

HeffAλ = EλAλ

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Optical absorption – Bethe-Salpeter equation

Bethe-Salpeter equation (BSE):

HeffAλ = EλAλ

Heffvv′cc′ = (ǫc − ǫv)δvv′δcc′ + 〈vc|v|v′c′〉 − 〈vc|W |v′c′〉

v ≡ Coulomb potential without the long-range part ǫv,c are G0W0 quasiparticle energies

macroscopic dielectric constant

ImεM (ω) =8π2

V

λ

∣∣∣∣∣

occ∑

v

unocc∑

c

Aλvc

〈v|p|c〉

ǫc − ǫv

∣∣∣∣∣

2

δ(ω−Eλ)

p ≡ dipole operator

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Optical absorption – Recipe for BSE

1 perform a DFT calculation

2 perform a G0W0 calculation

3 calculate 〈vc|v|v′c′〉 and 〈vc|W |v′c′〉

4 solve HeffAλ = EλAλ

5 calculate Im εM (ω)

Patrick Rinke (FHI) Excited States Berlin 2011 50 / 54

Optical absorption – response function

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0ω (eV)

0

10

20

30

40

50

60

Im ε

(ω)

Exp.BSETDLDA

absorption spectrum of silicon

from Sottile, Olevano, and Reining, PRL 91, 056402 (2003)

Patrick Rinke (FHI) Excited States Berlin 2011 51 / 54

Optical absorption – response function

2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0ω (eV)

0

10

20

30

40

50

60

Im ε

(ω)

Exp.BSE"GW"

absorption spectrum of silicon

BSE creates bound exciton

BSE shifts spectral weightPatrick Rinke (FHI) Excited States Berlin 2011 52 / 54

Band structures: photo-electron spectroscopy

- -

-

-

+

Photoemission

hn

GW

- -

-

Inverse Photoemission

-

-

hn

-

GW

- -

-+

Absorption

-

hn

BSETDDFT

Patrick Rinke (FHI) Excited States Berlin 2011 53 / 54

Acknowledgements

GW surface calculations

Christoph Freysoldt

GW for f -electrons

Hong JiangRicardo Gomez-Abal

G0W0 in FHI-aims

Xinguo Ren

scGW in FHI-aims

Fabio CarusoAngel Rubio

Support and Ideas

Matthias Scheffler

gwst

Rex Goby

FHI-aims

Volker Blumthe whole FHI-aims developer team

Patrick Rinke (FHI) Excited States Berlin 2011 54 / 54