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Efficient methods for computing
exchange-correlation potentials for
orbital-dependent functionals
Viktor N. Staroverov
Department of Chemistry, The University of Western Ontario,
London, Ontario, Canada
IWCSE 2013, Taiwan National University, Taipei, October 14β€’17, 2013
Orbital-dependent functionals
𝐸XC 𝜌 =
𝑓 πœ™π‘–
𝑑𝐫
Kohn-Sham orbitals
• More flexible than LDA and GGAs (can satisfy
more exact constraints)
• Needed for accurate description of molecular
properties
2
Examples
• Exact exchange
𝐸Xexact
1
𝜌 =−
4
𝑁
𝑑𝐫
𝑖,𝑗=1
∗
∗ ′
′
πœ‘
𝐫
πœ‘
𝐫
πœ‘
𝐫
πœ‘
𝐫
𝑖
𝑗
𝑗
𝑖
𝑑𝐫 ′
𝐫 − 𝐫′
same expression as in the Hartreeβ€’Fock theory
• Hybrids (B3LYP, PBE0, etc.)
• Meta-GGAs (TPSS, M06, etc.)
3
The challenge
Kohnβ€’Sham potentials corresponding to orbitaldependent functionals
𝑣XC
𝛿𝐸XC [{πœ™π‘– }]
𝐫 =
=?
π›ΏπœŒ(𝐫)
cannot be evaluated in closed form
4
Optimized effective potential (OEP)
method
Find 𝑣XC (𝐫) as the solution to the minimization
problem
𝛿𝐸total
=0
𝛿𝑣XC (𝐫)
OEP = functional derivative of the functional
5
Computing the OEP
Expand the Kohnβ€’Sham orbitals:
𝑛
πœ™π‘– 𝐫 =
π‘π‘˜π‘– πœ’π‘˜ (𝐫)
π‘˜=1
orbital basis functions
Expand the OEP:
π‘š
𝑣XC 𝐫 =
π‘π‘˜ π‘“π‘˜ (𝐫)
π‘˜=1
auxiliary basis functions
Minimize the total energy with respect to {π‘π‘˜π‘– } and {π‘π‘˜ }
6
Attempts to obtain OEP-X in finite basis sets
size
7
I. First approximation to the OEP:
An orbital-averaged potential (OAP)
Define operator 𝑒XC such that
𝛿𝐸XC [{πœ™π‘– }]
𝑒XC πœ™π‘– 𝐫 =
π›Ώπœ™π‘–∗ (𝐫)
The OAP is a weighted average:
𝑣XC 𝐫 =
𝑁
∗
πœ™
𝑖=1 𝑖 𝐫 𝑒XC πœ™π‘–
𝑁
∗
πœ™
𝑖=1 𝑖 (𝐫)πœ™π‘– 𝐫
𝐫
8
Example: Slater potential
Fock exchange operator:
𝛿𝐸Xexact
πΎπœ™π‘– 𝐫 ≡
π›Ώπœ™π‘–∗ (𝐫)
Slater potential:
1
𝑣S 𝐫 =
𝜌 𝐫
𝑁
πœ™π‘–∗ (𝐫)πΎπœ™π‘– (𝐫)
𝑖=1
9
Calculation of orbital-averaged
potentials
• by definition (hard, functional specific)
• by inverting the Kohnβ€’Sham equations
(easy, general)
10
Kohnβ€’Sham inversion
Kohnβ€’Sham equations:
1 2
− ∇ + 𝑣 + 𝑣H + 𝑣XC πœ™π‘– = πœ–π‘– πœ™π‘–
2
multiply by πœ™π‘–∗ ,
sum over i,
divide by 𝜌
𝜏𝐿
1
+ 𝑣 + 𝑣H + 𝑣XC =
𝜌
𝜌
𝑁
πœ–π‘– πœ™π‘–
2
𝑖=1
11
LDA-X potential via Kohn-Sham inversion
12
PBE-XC potential via Kohnβ€’Sham inversion
13
Removal of
oscillations
A. P. Gaiduk,
I. G. Ryabinkin, VNS,
JCTC 9, 3959 (2013)
14
Kohnβ€’Sham inversion for orbitalspecific potentials
Generalized Kohnβ€’Sham equations:
1 2
− ∇ + 𝑣 + 𝑣H + 𝑒XC πœ™π‘– = πœ–π‘– πœ™π‘–
2
same manipulations
𝜏𝐿
1
+ 𝑣 + 𝑣H + 𝑣XC =
𝜌
𝜌
𝑁
πœ–π‘– πœ™π‘–
2
𝑖=1
15
Example: Slater potential through
Kohnβ€’Sham inversion
1 2
𝛻 𝜌 𝐫 − 𝜏(𝐫) +
𝑣S 𝐫 = 4
𝜌 𝐫
𝑁
𝑖=1 πœ–π‘– |πœ™π‘–
(𝐫)|2
− 𝑣 𝐫 − 𝑣H (𝐫)
where
1
𝜏=
2
𝑁
|π›»πœ™π‘–
𝑖=1
|2
1 2
= 𝜏𝐿 + 𝛻 𝜌
4
16
Slater potential via Kohnβ€’Sham inversion
17
OAPs constructed by Kohnβ€’Sham inversion
18
Correlation potentials via Kohnβ€’Sham inversion
19
Kohnβ€’Sham inversion for a fixed set
of Hartreeβ€’Fock orbitals
Slater potential:
𝑣SHF =
−𝜏𝐿HF +
𝑁
HF HF |2
πœ–
𝑖=1 𝑖 |πœ™π‘–
𝜌HF
− 𝑣 − 𝑣HHF
𝑁
HF |2
πœ–
|πœ™
𝑖=1 𝑖 𝑖
𝜌HF
− 𝑣 − 𝑣HHF
But if πœ™π‘–OEP ≈ πœ™π‘–HF , then
𝑣XOEP ≈ 𝑣Xmodel =
−𝜏𝐿HF +
20
Dependence of KS inversion on orbital energies
21
II. Assumption that the OEP and HF
orbitals are the same
The assumption
πœ™π‘– = πœ™π‘–HF
leads to the eigenvalue-consistent orbitalaveraged potential (ECOAP)
𝑣XECOAP
=
𝑣SHF
1
+ HF
𝜌
𝑁
(πœ–π‘– −
πœ–π‘–HF )
HF 2
πœ™π‘–
𝑖=1
22
ECOAP ≈ KLI ≈ LHF
23
Calculated exact-exchange (EXX) energies
𝐸EXX − 𝐸OEP , mEh
m.a.v.
KLI
ELP=LHF=CEDA
ECOAP
2.88
2.84
2.47
Sample: 12 atoms from He to Ba
Basis set: UGBS
A. A. Kananenka, S. V. Kohut, A. P. Gaiduk, I. G. Ryabinkin, VNS,
JCP 139, 074112 (2013)
24
III. Hartreeβ€’Fock exchangecorrelation (HFXC) potential
An HFXC potential is the 𝑣XC (𝐫) which reproduces
a HF density within the Kohnβ€’Sham scheme:
1 2
− ∇ + 𝑣 𝐫 + 𝑣H 𝐫 + 𝑣XC 𝐫 πœ™π‘– (𝐫) = πœ–π‘– πœ™π‘– (𝐫)
2
That is, 𝑣XC (𝐫) is such that
𝑁
𝜌 𝐫 =
𝑁
πœ™π‘– 𝐫
𝑖=1
2
πœ™π‘–HF
=
𝐫
2
= 𝜌HF (𝐫)
𝑖=1
25
Inverting the Kohn–Sham equations
Kohnβ€’Sham equations:
1 2
− ∇ + 𝑣 + 𝑣H + 𝑣XC πœ™π‘– = πœ–π‘– πœ™π‘–
2
multiply by πœ™π‘–∗ ,
sum over i,
divide by 𝜌
𝜏𝐿
1
+ 𝑣 + 𝑣H + 𝑣XC =
𝜌
𝜌
𝑁
πœ–π‘– πœ™π‘–
2
𝑖=1
local ionization
potential
26
Inverting the Hartree–Fock equations
Hartreeβ€’Fock equations:
1 2
− ∇ + 𝑣 + 𝑣H + 𝐾 πœ™π‘–HF = πœ–π‘–HF πœ™π‘–HF
2
same manipulations
𝜏𝐿HF
𝜌HF
+ 𝑣 + 𝑣H +
𝑣SHF
1
= HF
𝜌
𝑁
πœ–π‘–HF πœ™π‘–HF
2
𝑖=1
Slater potential built
with HF orbitals
27
Closed-form expression for
the HFXC potential
HF
𝑣XC
1
HF
= 𝑣S +
𝜌
𝑁
𝑖=1
1
2
πœ–π‘– |πœ™π‘– | − HF
𝜌
𝑁
𝑖=1
HF HF 2
πœ–π‘– πœ™π‘–
𝜏 HF 𝜏
+ HF −
𝜌
𝜌
Here
𝜌 = 𝜌HF , but πœ™π‘– ≠ πœ™π‘–HF , πœ–π‘– ≠ πœ–π‘–HF , and 𝜏 ≠ 𝜏 HF
We treat this expression as a model potential within the
Kohnβ€’Sham SCF scheme.
Computational cost: same as KLI and Beckeβ€’Johnson (BJ)
28
HFXC potentials are practically exact OEPs!
Numerical OEP: Engel et al.
29
30
31
HFXC potentials can be easily computed
for molecules
Numerical OEP: Makmal et al.
32
Energies from exchange potentials
𝐸EXX − 𝐸OEP , mEh
m.a.v.
KLI
LHF
BJ
1.74
1.66
5.30
Basisset OEP
0.12
HFXC
0.05
Sample: 12 atoms from Li to Cd
Basis set for LHF, BJ, OEP (aux=orb), HFXC: UGBS
KLI and true OEP values are from Engel et al.
I. G. Ryabinkin, A. A. Kananenka, VNS, PRL 139, 013001 (2013)
33
Virial energy discrepancies
For exact OEPs,
𝐸vir − 𝐸EXX = 0,
where
𝐸vir = ∫ 𝑣X 𝐫 3𝜌 𝐫 + 𝐫 ⋅ ∇𝜌(𝐫) 𝑑𝐫
𝐸vir − 𝐸EXX , mEh
KLI
m.a.v. 438.0
LHF
BJ
629.2 1234.1
Basis-set
HFXC
OEP
1.76
2.76
34
HFXC potentials in finite basis sets
35
Hierarchy of approximations to
the EXX potential
𝑣X
1
HF
= 𝑣S + HF
𝜌
𝑁
πœ–π‘– − πœ–π‘–HF πœ™π‘–HF
𝑖=1
HF − 𝜏
𝜏
2
+
𝜌HF
OAP
ECOAP
HFXC
36
Summary
• Orbital-averaged potentials (e.g., Slater) can be
constructed by Kohnβ€’Sham inversion
• Hierarchy or approximations to the OEP:
OAP (Slater) < ECOAP < HFXC
• ECOAP Slater potential ο‚» KLI ο‚» LHF
• HFXC potential ο‚» OEP
• Same applies to all occupied-orbital functionals
37
Acknowledgments
• Eberhard Engel
• Leeor Kronik
for OEP
benchmarks
38
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