A incomplete investigation of “first principles ...xinguo/talks/jiang_cdft-coffeetalk.pdf · How...
Transcript of A incomplete investigation of “first principles ...xinguo/talks/jiang_cdft-coffeetalk.pdf · How...
How much we know about U? A incomplete investigation of “first
principles” determination of the Hubbard U
Hong Jiang
VB CBEF
ULDA+U
VB CB
LDA
EF
Outline• LDA+U method
• Firstprinciples determination of U
• Constrained DFT calculations of U in WIEN2k
• Linearresponse approach for U in WIEN2k and PWscf
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Failure of LDA for fractional charge systems
Perdew et al. (1982); Mori Sanchez, Cohen and Yang, PRL 77, 115123(2008)
Convex (e.g. LDA/GGA)
Concave (HF)
exact
More localized states have more severe selfinteraction (delocalization) error
Openshell d or felectron systems: fractional occupation appears even at integer number
of electrons
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Challenge of d or felectron systems
The error due to fractional charges is more severe for localized dor fstates
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LDA+U : a correction from Hubbard Model
Anisimov et al. (1993); Dudarev et al. (1998)
Hubbard model for local interaction:
HartreeFock approximation:
For integer occupation
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Assumptions within LDA+U
Anisimov, Aryasetiawan and Lichtenstein, JPCM (1997)
Identification of a local subsystem for which a correction is made
meanfield approximation for the Hubbard model is adequate to describe
manybody interaction in the local subsystem
For the doublecounting correction, the LDA interaction energy for the
local subsystem can be described by the same parameters U and J
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U : local screened Coulomb interaction
Ce atom
F0(Ce, LDA)=24.7 eV
Where are screening from? Screening of itinerant states Relaxation of dwave functions due to changing N
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Direct approaches: Constrained RPA
Aryasetiawan et al. PRB74, 115106 (2006); Miyake and Aryasetiawan, PRB 77, 085122 (2008)
LMTO Localized Wannier func.
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cRPA: How to separate localized states from others
Aryasetiawan et al. PRB74, 115106 (2006); Miyake and Aryasetiawan, PRB 77, 085122 (2008)
(from Aryasetiawan's talk)
spCB
spVB
dstates
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Supercell constrained DFT approaches
dndn1 dn+1
U and J consistent with doublecounting corrections
(spinpolarized)
(spinunpolarized)
By second order derivative:
By second order difference
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“Hard” cDFT method
dndn1 dn+1
U=E(dn+1) + E(dn1)2E(dn) Anisimov and Gunnarsson (1991)
Recipe( NiO as an example)
A supercell of NiO is built and one Ni atom is treated as an ``impurity'', on which the
occupation of the 3d orbital is constrained to nd
Hopping integrals connecting the impurity 3d orbitals and others orbitals are put zero (In
WIEN2k: the 3d orbital on the impurity atom is removed from LAPW basis)
U is calculated from the total energy difference with respect to changing nd
As nd varies, the system is kept neutral
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cDFT in WIEN2k: charge neutralizationVBN: neutralization by changing VB electron numbers
BCN: neutralization by constant background charges
Fe
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“Hard” cDFT method: main features
Local orbital occupation is well defined, easy to implement in augmentationbased (LMTO,
LAPW etc.) approaches
By removing 3d orbital on the impurity from the LAPW basis, the basis set becomes
incomplete
Inconsistency between the determination of U and the use of U in LDA+U: Local occupation
is defined differently
Not universally applicable
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CDFT: Intersite interaction
Applying LDA+U correction to nonimpurity target atoms to reduce
intersite interaction
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“Soft” cDFT method: internally consistent?
dndn1 dn+1
Nakamura et al. Phys. Rev. B 74, 235113 (2006)
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“Soft” cDFT: linearresponse approach
Cococcioni and de Gironcoli (2005)
U is defined here as 2nd partial derivative of E with occupation on all
othersites fixed A noninteracting (freeelectron) contribution is subtracted
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“Soft” cDFT: linearresponse approach
Cococcioni and de Gironcoli (2005)
Background contributions
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Ongoing or “to do”......
What is the cause for the difference between LAPW and LMTOASA cDFT
results?
Systematic comparison of direct (cRPA), “hard” and “soft” cDFT within the
same methodology
“soft”cDFT approaches are internally consistent and generally applicable,
but how strongly does it depends on the definition of the local projector operator (atomic, orthogonalized atomic, Wannier ... )
Is it possible to get rid of such dependence by consistently using same local
projector for cDFT and LDA+U calculations? ......