Diffusive Molecular Dynamics - Ju Li
Transcript of Diffusive Molecular Dynamics - Ju Li
Diffusive Molecular Dynamics
Ju Li, Bill Cox, Tom Lenosky,Ning Ma, Yunzhi Wang
Ohio State University
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Traditional Molecular Dynamics
• Traditional MD numerically integrates Newton’s equation of motion over 3N degrees of freedom, the atomic positions:
• It is difficult to reach diffusive time scales using traditional MD due to timestep (~ ps / 100), which needs to resolve atomic vibrations.
{ }, 1..i i N=x
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Diffusive MD: The Idea
Ferris wheel seen with long camera exposure time
Variational Gaussian Method
Lesar, Najafabadi and Srolovitz, Phys. Rev. Lett. 63 (1989) 624.
{ }, , 1..i i i Nα =x
DMD
ci: occupation probability(vacancy, solutes)
Define µi for each atom,to drive diffusion
{ }, , , 1..i i i i Nα =x c
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( ) ( ) ( )3 23 2 2 2
0 0 01
Gibbs-Bogoliubov Free Energy Bound:
1 exp exp | |2
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(| |, , )
Nji
i i i j j j i j i ji i j
B
i j i j
F F U U u d d
k T
w
αα α απ π
α α
∞ ∞
−∞ −∞= ≠
′ ′ ′ ′ ′ ′≤ + − = − − − − −
+
−
∑∑ ∫ ∫ x x x x x x x x
x x
2
1
ln
2Thermal wavelength:
Ni T
i
TB
e
mk T
απ
π=
Λ
Λ =
∑
Variational Gaussian Method
{xi,αi}true free energy
VG free energy
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Comparison with Monte Carlo
Lesar, Najafabadi and Srolovitz, Phys. Rev. Lett. 63 (1989) 624.
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DMD thermodynamics
( ) ( )2
1 1
1 3(| |, , ) ln ln 1 ln 12 2
N Ni
i j i j i j B i i i i ii i j i
F c c w k T c c c c ce
αα απ= ≠ =
Λ≤ − + + + − −
∑∑ ∑x x
{ }Add occupation order parameters to sites: , , , 1..i i i i Nα =x c
VG view DMD view
01
=
c
10
=
c
10
2
1 1
The chemical potential for each atom is easily derived:
1 3(| |, , ) ln ln2 2 1
N Ni i
i j i j i j Bi i j ii i
A cc w k Tc e c
αµ α απ= ≠ =
∂ Λ = = − + + ∂ − ∑∑ ∑x x
DMD kinetics
nearest-neighbor network
( )1
1 , if and are nearest neighbors2
0 otherwise
Ni
ij j ij
i j
ij
c kt
c ck i j
k
µ µ=
∂= −
∂
+ − =
∑
2B 0
calibrate against experimental diffusivity:Dk
k T a Z=
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• DMD is atomistic realization of regular solution model, with gradient thermo, long-range elastic interaction, and short-range coordination interactions all included.
• DMD kinetics is “solving Cahn-Hilliard equation on a moving atom grid”, with atomic spatial resolution, but at diffusive timescales.
• The “quasi-continuum” version of DMD can be coupled to well-established diffusion-microelasticity equation solvers such as phase-field method.
• No need to pre-build event catalog. Could be competitiveagainst kinetic Monte Carlo.