Extending the domain of quantum mechanical simulations with HPCx: Melting Dario Alfè University...

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Extending the domain of quantum mechanical simulations with HPCx: Melting Dario Alfè University College London

Transcript of Extending the domain of quantum mechanical simulations with HPCx: Melting Dario Alfè University...

Page 1: Extending the domain of quantum mechanical simulations with HPCx: Melting Dario Alfè University College London.

Extending the domain of quantum mechanical simulations

with HPCx: Melting

Dario AlfèUniversity College London

Page 2: Extending the domain of quantum mechanical simulations with HPCx: Melting Dario Alfè University College London.

Why Melting ?

• The Earth’s core is mainly iron

• Melting temperature of Fe at ICB

• Constraint on the temperature of the core

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Melting

Free energy approach

Coexistence approach

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Free energy approach

liquid solidG (P,T) G (P,T)

T 100 K G 10 meV/atom

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Calculating free energies

Thermodynamic integration:

1

ref ref

0

F F d U U

λ refU (1 λ)U λU

BU(R)/ TB 3N

V

1F(V,T) T ln dR e

N!kk

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Size and k-points tests

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Lidunka Vočadlo & Dario Alfè, PRB, 65, 214105 (2002)

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The coexistence approach

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• Density Functional Theory • Generalized Gradient Approximation (PW91)

• VASP code (Kresse and Furthmuller, PRB 54, 11169 (1996))

• USPP (130 eV PW-cutoff) • Finite temperature Fermi smearing • K-points sampling• Efficient charge density extrapolation (Alfe`, Comp.

Phys. Comm. 118, 31 (1999))

Ab-initio technical details

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Scaling tests (Al, 1000 atoms)

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512 atoms ()(~2 weeks HPCx, 64 PEs)

1000 atoms()(~3 weeks HPCx, 128 PEs)

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Dario Alfè, Phys. Rev. B, 68, 064423 (2003)

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512 atoms (2x2x1)(~4 weeks SUN-SPARC, 16 PEs)

1728 atoms()(~7 months SUN-SPARC, 16 PEs)

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Conclusions

• Coexistence of phases for melting is now possible even with first principles techniques (though still very expensive).

• Next step: Iron ? (One order of magnitude more expensive than Aluminium).

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