Sampling the Brillouin-zone:Andreas EICHLER Institut fur Materialphysik and Center for Computational Materials Science Universit t Wien, Sensengasse 8, A-1090 Wien, Austria a
b-initio
ackage ienna imulation
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Overviewintroduction k-point meshes Smearing methods What to do in practice
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IntroductionFor many properties (e.g.: density of states, charge density, matrix elements, response functions, . . . ) integrals (I) over the Brillouin-zone are necessary: I
1 BZ
F nk
dk
To evaluate computationally weighted sum over special k-points integrals
1 BZ
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kik
BZ
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k-points meshes - The idea of special pointsChadi, Cohen, PRB 8 (1973) 5747.
function f k with complete lattice symmetry introduce symmetrized plane-waves (SPW): Am k
R
ekRCm
sum over symmetry-equivalent R Cm SPW
shell of lattice vectors
develope f k in Fourier-series (in SPW) f k
Cm
f0
m 1
1
f m Am k
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evaluate integral (=average) over Brillouin-zone f with: 2 3
2 3
BZ
f k dk
BZ
Am k dk
0
m
12
taking n k-points with weighting factors k so thati 1
f = weighted sum over k-points for variations of f that can be described within the shell corresponding to CN .
f
f0
ki A m
n
ki
0
m
1
N
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Monkhorst and Pack (1976):Idea: equally spaced mesh in Brillouin-zone. Construction-rule: k prs ur bi qr u p b1
ur b2 r
us b3 qr
2r qr 1 2qr
12
reciprocal lattice-vectors determines number of k-points in r-direction
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Example: quadratic 2-dimensional lattice q1 q2 4 16 k-points IBZ)
r b"
only 3 inequivalent k-points ( 4 4 81 BZ
0
k k k r b#$ & % !
k1 k2 k3
1 8 3 8 3 8
1 8 3 8 1 8
1 2 3 k1
1 4 1 4 1 2 1 4F
IBZ BZk21 2F
BZ
F k dk
1 4F
k3
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Interpretation: representation of functionF k on a discrete equally-spaced mesh
En 0
an cos 2nk
N
-
0
kmore Fourier-components higher accuracy
density of mesh
Common meshes : Two choices for the center of the mesh centered on ( belongs to mesh).
centered around . (can break symmetry !!)
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Algorithm: calculate equally spaced-mesh shift the mesh if desired apply all symmetry operations of Bravaislattice to all k-points extract the irreducible k-points ( calculate the proper weighting IBZ)'
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Smearing methodsProblem: in metallic systems Brillouin-zone integrals over functions that are discontinuous at the Fermi-level. high Fourier-components dense grid is necessary. Solution: replace step function by a smoother function. Example: bandstructure energynk
k nk nk (
with: x
1 x 0 x 2
0 0
E e4
nk
k nk f
) 0
nk
1
-
0
k
necessary: appropriate function f
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f equivalent to partial occupancies.B RILLOUIN - ZONE Page 10
Fermi-Dirac function
f
nk
exp
nk
1
1
consequence: energy is no longer variational with respect to the partial occupacies f . 1 2 3 F S f
53
2
E
S fnn
F free energy. new variational functional - dened by (1). S f
entropy of a system of non-interacting electrons at a nite temperature T.
f ln f
1
f ln 1
f
6
7
kB T
smearing parameter. can be interpreted as nite temperature via (3). calculations at nite temperature are possible (Mermin 1965)A. E ICHLER , S AMPLINGTHE
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Consistency: 1 2 3 4 F E S fnn
S f
f ln f
1
f ln 1
f
kB T F8
6
7
fn E fn S f E fn
fnn
N9
0 ln 1 f f
1
4
5
S fn
0 ln 1
@ A
5
2
6
ln f
1
f
1
7
@
7
8
n 1 fn
6
7
n
ln 1 fnfn
0
8
@ A
9
exp fn
n
11
@
B
C
9
exp
nk E
1
@
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Gaussian smearingbroadening of energy-levels with Gaussian function. f becomes an integral of the Gaussian function: f nk
1 1 28
nk erf
no analytical inversion of the error-function erf exists entropy and free energy cannot be written in terms of f . S
9
1 2 G
exp
2
has no physical interpretation. variational functional F differs from E 0 . forces are calculated as derivatives of the variational quantity (F ). not necessarily equal to forces at E 0 . A. E ICHLER , S AMPLING
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Improvement: extrapolation to
0.
@
1 2 3
F F S
E 0
2 S
Q P
E
1 1
3
4
E
F
2
P
E 0
21 2
P
@
4
E 0
E
F
E
P
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Method of Methfessel and Paxton (1989)Idea: expansion of stepfunction in a complete set of orthogonal functions term of order 0 = integral over Gaussians generalization of Gaussian broadening with functions of higher order.
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f0 x fN x SN x
1 2
1
erf xN
with:
advantages: deviation of F from E 0 only of order 2+N in extrapolation for
f0 x
An
1 2 AN H2N 1n n!4n R
m 1
Am H2m xe
1
xe
x2
x2
HN : Hermite-polynomial of order N
0 usually not necessary, but also possible: E
@
E 0
N 2
1
N
1F
E
P
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The signicance of N and MP of order N leads to a negligible error, if X is representable as a polynomial of degree 2N around F . linewidth can be increased for higher order to obtain the same accuracy entropy term (S S
n SN fn ) describes deviation of F from E .
if S few meV F then E P Q
forces correct within that limit.
in practice: smearings of order N=1 or 2 are sufcient
E
E 0.
P
P Q
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Linear tetrahedron method
Idea: 1. dividing up the Brillouin-zone into tetrahedra 2. Linear interpolation of the function to be integrated Xn within these tetrahedra 3. integration of the interpolated function Xn
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ad 1. How to select mesh for tetrahedra map out the IBZ use special points
r bU
r bW
3 1 3
1 3 r 1 b
4 1 4
1 4 r 1 b
V
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ad 2. interpolation
Seite 1 von 1
Xn k
c j k Xn k jj
j .......... k-points
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ad 3. k-space integration: simplication by Blochl (1993)
remapping of the tetrahedra onto the k-points n j 1 BZ dkc j k f n k
BZ
effective weights n j for k-points. k-space summation:
n j Xnnj
kj
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Drawbacks: tetrahedra can break the symmetry of the Bravaislattice at least 4 k-points are necessary must be included linear interpolation under- or overestimates the real curve
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Corrections by Bl chl (1993) oIdea: linear interpolation under- or overestimates the real curve for full-bands or insulators these errors cancel for metals: correction of quadratic errors is possible: kn j DT
T
1 40 DT
F jnj 1
4
kn
corners (k-point) of the tetrahedronT DOS for the tetrahedron T at F .
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Result: best k-point convergence for energy forces: with Bl chl corrections the new effective partial occupancies do not minimize the o groundstate total energy variation of occupancies nk w.r.t. the ionic positions would be necessary with US-PP and PAW practically impossible
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Convergence tests
(from P.Blochl, O. Jepsen, O.K. Andersen, PRB 49,16223 (1994).)
bandstructure energy of silicon: conventional LT -method vs. LT+Bl chl corrections o bandstructure energy vs. k-point spacing :
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What to do in practiceenergy/DOS calculations: linear tetrahedron method with Bl chl corrections o ISMEAR=-5 calculation of forces: semiconductors: Gaussian smearing (ISMEAR=0; SIGMA=0.1) metals : Methfessel-Paxton (N=1 or 2) always: test for energy with LT+Bl chl-corr. o
in any case: careful checks for k-point convergence are necessary
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The KPOINTS - le:1> 2> 3> 4> 5> k-points for a metal 0 Gamma point 9 9 9 0 0 0
1st line: comment 2nd line: 0 ( automatic generation)
3rd line: Monkhorst or Gammapoint (centered) 4th line: mesh parameter 5th line: 0 0 0 (shift)
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mesh parameter determine the number of intersections in each direction longer axes in real-space
shorter axes in k-space
less intersections necessary for equally spaced mesh
Consequences: molecules, atoms (large supercells) 1 1 1 is enough.'
2-D Brillouin-zone) surfaces (one long direction x y 1 for the direction corresponding to the long direction.
typical values (never trust them!):
metals: semiconductors:
9
9 4
9 /atom
4
4 /atom
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Example - real-space/ reciprocal cellr bY
r bq
44ige h f
0c a db`
k1 k2 k3 r bX
24
k1 k2
0t s dbr
r bp
x v iwu
IBZ BZ
IBZ BZ
doubling the cell in real space halves the reciprocal cell zone boundary is folded back to same sampling is achieved with halved mesh parameter
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Example - hexagonal cellbefore after symmetrization
shifted to G
r y
r
in certain cell geometries (e.g. hexagonal cells) even meshes break the symmetry symmetrization results in non equally distributed k-points Gamma point centered mesh preserves symmetryA. E ICHLER , S AMPLINGTHE
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Convergence testswith respect to and number of k-points in the IBZ
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G.Kresse, J. Furthm ller, Computat. Mat. Sci. 6, 15 (1996). uB RILLOUIN - ZONE Page 31