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The Materials World
NANO266 2
Molecules
Isolated gas phase
Typically use localized basis functions, e.g.,
Gaussians
Everything else (liquids,
amorphous solids, etc.)
Too complex for direct QM!
(at the moment)
But can work reasonable
models sometimes
Crystalline solids
Periodic infinite solid
Plane-wave approaches
What is a crystal?
A crystal is a time-invariant, 3D arrangement of atoms or molecules on a lattice.
NANO266
Perovskite SrTiO3
The “motif”
repeated on each point in the cubic lattice below…
3
Translational symmetry
All crystals are characterized by translational symmetry
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t = ua+ vb+wc, u,v,w ∈ Z1D 2D (single layer MoS2)
3D
The 14 3D Bravais Lattices
NANO 106 - Crystallography of Materials by Shyue Ping Ong - Lecture 2
P: primitive C: C-centered I: body-centered F: face-centered (upper case for 3D)
a: triclinic (anorthic) m: monoclinic o: orthorhombic t: tetragonal h: hexagonal c: cubic
3D unit cells
Infinite number of unit cells for all 3D lattices
Always possible to define primitive unit cells for non-primitive lattices, though the full symmetry may not be retained.
NANO 106 - Crystallography of Materials by Shyue Ping Ong - Lecture 2
Conventional cF cell Primitive unit cell
The Reciprocal Lattice
For a lattice given by basis vectors a1, a2 and a3, the reciprocal lattice is given basis vectors a1*, a2* and a3* where:
NANO 106 - Crystallography of Materials by Shyue Ping Ong - Lecture 2
a1* = 2π a2 ×a3
a1.(a2 ×a3 )
a2* = 2π a1 ×a3
a1.(a2 ×a3 )
a3* = 2π (a1 ×a2 )
a1.(a2 ×a3 )
ai*a j = 2πδij
Reciprocal lattice
Translation vectors in the reciprocal lattice is given by:
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G = ha1* + ka2
* + la3*
Direct lattice Reciprocal lattice Simple Cubic Simple Cubic Face-centered cubic (fcc) Body-centered cubic (bcc) Body-centered cubic (bcc) Face-centered cubic (fcc) Hexagonal Hexagonal
Periodic Boundary Conditions
Repeat unit cell infinitely in all directions.
What does this mean for our external potential (from the nuclei)?
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Electron in a periodic potential
For an electron in a 1D periodic potential with lattice vector a, we have
For any periodic function, we may express it in terms of a Fourier series
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H = −12∇+V (x)
where V (x) =V (x +ma).
V (x) = Vnei2πanx
n=−∞
∞
∑
Bloch’s Theorem
For a particle in a periodic potential, eigenstates can be written in the form of a Bloch wave
Where u(r) has the same periodicity as the crystal and k is a vector of real numbers known as the crystal wave vector, n is known as the band index.
For any reciprocal lattice vector K, , i.e., we only need to care about k in the first Brillouin Zone
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ψn,k (r) = eik.run,k (r)
Plane wave
ψn,k+K (r) =ψn,k (r)
Plane waves as a basis
Any function that is periodic in the lattice can be written as a Fourier series of the reciprocal lattice Recall that from the Bloch Theorem, our wave function is of the form Where u(r) has the same periodicity as the crystal and k is a vector of real numbers known as the crystal wave vector, n is known as the band index.
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ψn,k (r) = eik.run,k (r)
f (x) = cnei2πanx
n=−∞
∞
∑
Reciprocal lattice vector in 1D
Plane waves as a basis
Let us now write u(r) as an expansion
Our wave function then becomes
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un,k (r) = cGn,keiGr
G∑
ψn,k (r) = cGn,kei(k+G).r
G∑
Using the plane waves as basis
Plane waves offer a systematic way to improve completeness of our solution
Recall that for a free electron in a box,
Corresponding, each plane wave have energy
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ψn,k (r) = cGn,kei(k+G).r
G∑
Infinite sum over reciprocal space
ψ(r) = eik.r and the corresponding energy is E = !2
2mk2
E = !2
2mk+G 2
Energy cutoff
Solutions with lower energy are more physically important than solutions with higher energies
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Ecut =!2
2mGcut2
ψn,k (r) = cGn,kei(k+G).r
k+G<Gcut
∑
Convergence with energy cutoff
The same energy cutoff must be used if you want to compare energies between calculations, e.g., if you want to compute:
Cu (s) + Pd (s) -> CuPd(s)
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Pseudopotentials
Problem: Tightly bound electrons have wavefunctions that oscillate on very short length scales => Need a huge cutoff (and lots of plane waves). Solution: Pseudopotentials to represent core electrons with a smoothed density to match various important physical and mathematical properties of true ion core NANO266 19
ψn,k (r) = cGn,kei(k+G).r
G∑
Types of pseudopotentials (PPs)
Norm-conserving (NC) • Enforces that inside cut-off radius, the norm of the pseudo-wavefunction
is identical to all-electron wavefunction.
Ultrasoft (US) • Relax NC condition to reduce basis set size further
Projector-augmented wave (PAW) • Avoid some problems with USPP • Generally gives similar results as USPP and all-electron in many
instances.
NANO266 20 Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method, Phys. Rev. B, 1999, 59, 1758–1775.
How do you choose PPs?
Sometimes, several PPs are available with different number of “valence” electrons, i.e., electrons not in the core.
Choice depends on research problem – if you are studying problems where more (semi-core) electrons are required, choose PP with more electrons
But more electrons != better results! (e.g., Rare-earth elements)
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Born–von Karman boundary condition
Consider large, but finite crystal of volume V with edges
Born-von Karman boundary condition requires
Since we have Bloch wavefunctions,
Therefore, possible k-vectors compatible with cyclic boundaries are given by:
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N1t1, N2t2, N3t3
ψ(r+ N1t1) =ψ(r+ N2t2 ) =ψ(r+ N3t3) =ψ(r)
eikN1t1 = eikN2t2 = eikN3t3 =1
k = m1N1g1 +
m2
N2
g2 +m3
N3
g3
Integrations in k space
For counting of electrons in bands and total energies, etc., need to sum over states labeled by k Numerically, integrals are performed by evaluating function at various points in the space and summing them.
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f = Vcell(2π )3
f (k)dkBZ∫
f = 1Nk
f (k)k∑
Choice of k-points
1. Sampling at one point (Baldereschi point, or Gamma point)
2. Monkhorst-Pack – Sampling at regular meshes
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Important things to note about k-point convergence
Symmetry reduces integrals to be performed -> Irreducible Brillouin Zone
k-point mesh is inversely related to unit cell volume (larger unit cell volume -> smaller reciprocal cell volume)
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k-point sampling in metals
BZ in metals are divided into occupied and unoccupied regions by Fermi surface, where the integrated functions change discontinuously from non-zero to zero. => Extremely dense k-point mesh needed for integration
Algorithmic solutions
• Tetrahedron method. Use k points to define a tetrahedra that fill reciprocal space and interpolate. Most widely used is Blochl’s version.
• Smearing. Force the function being integrated to be continuous by “smearing” out the discontinuity, e.g., with the Fermi-Dirac function or the Methfessel and Paxton method.
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Fermi-surface of Copper (Cu), the color codes the inverse effective mass of the electrons, large effective masses are represented in red, from A. Weismann et al., Science 323, 1190 (2009)