cks cambridge talk 2006-11-29 - onetep
TRANSCRIPT
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Chris-Kriton Skylaris
Calculations on metallic systems with ONETEP
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2EDFT, ONETEP Masterclass 2015
Density matrix DFT
DFT energy with molecular orbitals
DFT energy with density matrix
Density matrix
DensityMolecular orbitals
Localised orbitals
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EDFT, ONETEP Masterclass 2015 3
C.-K. Skylaris, P. D. Haynes, A. A. Mostofi and M. C. Payne, J. Chem. Phys. 122, 084119 (2005).
C.-K. Skylaris, P. D. Haynes, A. A. Mostofi and M. C. Payne, Phys. Stat. Sol. (b) 243, 973 (2006).
C.-K. Skylaris, A. A. Mostofi, P. D. Haynes, O. Dieguez, M. C. Payne, Phys. Rev. B 66, 035119 (2002).
The ONETEP linear-scaling approach• Use a minimal number of but optimise these, like K
• Linear-scaling DFT with the same level of basis set accuracy as conventional cubic-scaling DFT
Density kernel
Non-orthogonal Generalised Wannier Functions (NGWFs)
• NGWFs confined to spherical regions• Density kernel K sparse by truncation
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• Conventional DFT not suitable for materials with small or no band gap
• Small electronic excitations due to finite temperature need to be accounted for
4EDFT, ONETEP Masterclass 2015
DFT for metals
E
Pt
HOMO-LUMO gap
insulator metal
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5EDFT, ONETEP Masterclass 2015
Canonical ensemble electronic theory
• Wavefunction or density operator
where ΓN0 is the equilibrium canonical
ensemble density operator
• Helmholtz Free Energy (A=E-TS)
• Variational principle
• Ensemble density operator
• Energy (E)
Pure state (isolated system at T=0)
Mixed state (part of a larger system at finite T)
• Variational principle
Entropy
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• Finite temperature formulation by Mermin combines canonical ensemble statistical mechanics with DFT
• Mermin-Hohenberg-Kohn Helmholtz free energy functional
6EDFT, ONETEP Masterclass 2015
Finite temperature DFT
N. D. Mermin, Phys. Rev. 137, A1441 (1965)
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7EDFT, ONETEP Masterclass 2015
Kohn-Sham finite temperature DFT
• We can develop a Kohn-Sham approach based on Mermin’s finite temperature DFT
• Helmholtz Kohn-Sham free energy functional
• Electronic density
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• Molecular orbital occupancies follow Fermi-Dirac distribution
for molecular orbital ψi with energy εi
8EDFT, ONETEP Masterclass 2015
Kohn-Sham finite temperature DFT
• In the canonical ensemble µ is determined by conserving the number of electrons
• Electronic entropy
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9EDFT, ONETEP Masterclass 2015
Free energy with orthogonal orbitals
• Non-diagonal occupancies
The Helmholtz free energy • Eigenvalue equation for energies
canonical orbitals
non-canonical orbitals
•N. Marzari, D. Vanderbilt and M. C. Payne, Phys. Rev. Lett. 79, 1337 (1997)
•C. Freysoldt, S. Boeck, and J. Neugebauer, Phys. Rev. B, 79, 241103(R) (2009).
becomes
Efficient methods to minimise the free energy using non-canonical orbitals
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10EDFT, ONETEP Masterclass 2015
Free energy with non-orthogonal orbitals
canonical orbitals
NGWFs
Molecular orbitals (orthogonal)
ONETEP NGWFs(non-orthogonal)
• Eigenvalue equation to solve for orbital energies (and occupancies)
canonical orbitals
non-canonical orbitals
• Free energy
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Electronic free energy optimisation in ONETEP
11EDFT, ONETEP Masterclass 2015
Iteratively improve Hαβ
Converged?
Iteratively improve {φα}
Converged?
Guess Hαβ and {φα}
finished
Yes
No
No
Yes
A. Ruiz-Serrano and C.-K. Skylaris, J. Chem. Phys. 139, 054017 (2013)
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12EDFT, ONETEP Masterclass 2015
Inner loop: Hamiltonian matrix optimisation
F-D occupancies
Density
F-D Hamiltonian
Search direction
Energy minimisation
MO energies
m m+1
At iteration m
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13EDFT, ONETEP Masterclass 2015
Outer loop: NGWF optimisation
Conjugate gradients approach. Computation of gradient at iteration l:
1) Gradient
2) MO orthonormalityconstraint
3) Make covariant
4) Occupancy preconditioning
5) Kinetic energy preconditioning
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14EDFT, ONETEP Masterclass 2015
ri centre of Di(r)φα localisation sphere
In the psinc basis set the gradient is delocalised
Re-localise by truncating outside localisation region of φα
Use this gradient to iteratively optimise the NGWFs with a conjugate gradients approach
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15EDFT, ONETEP Masterclass 2015
B (GPa) L0 (Å)
CASTEP 93.57 3.79
ONETEP 93.51 3.79
Calculations on bulk Cu
ONETEP settingsK.E. cutoff: 900 eVKBT: 0.1 eVxc: RPBENGWF radius: 4 Å4500 energy levels (bands)
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16EDFT, ONETEP Masterclass 2015
Au cubo-octahedral nanoparticles•Optimisation of NGWFs•Convergence
Electron density isosurface of Au1415
• Psinc kinetic energy cut-off: 950 eV• XC functional: RPBE• NGWF radii: 9.45 a0
J. Kleis, …, J. K. Norskøv, et al, Catal. Lett. 141, 1067 (2011)
Reference: Au13 to Au1415, run on 32768 cores
(run on 192 cores)
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17EDFT, ONETEP Masterclass 2015
Performance and scaling with the number of atoms
• Most operations are linear-scaling operations such as the construction of the (sparse) Hamiltonian matrix
• But diagonalisation of the Hamiltonian is a cubic-scaling operation• Calculations with thousands of atoms are possible due to the minimal size of matrices in ONETEP and the use of parallel eigensolvers
•120 cores (2.4 GHz Intel Westmere)
•ScaLAPACK parallel eigensolver
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18EDFT, ONETEP Masterclass 2015
Ongoing work: Removing the diagonalization bottleneck
• Fermi Operator Expansion (FOE) techniques: Computing the density matrix as a matrix series expansion of the Hamiltonian matrix
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19EDFT, ONETEP Masterclass 2015
EDFT documentation on the ONETEP website
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Acknowledgements
20
SouthamptonJolyon AaronsAlvaro Ruiz-Serrano
EDFT, ONETEP Masterclass 2015
Iridis 4