(ab initio) pathintegral molecular dynamicsluca/course_tu/05_path... · the double slit experiment...
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(Ab initio) pathintegral Molecular Dynamics
The double slit experiment
Sum over paths:
Suppose only two paths:
interference
The double slit experiment
● Introduce of a large number of intermediate gratings, each containing many slits.
● Electrons may pass through any sequence of slits before reaching the detector
● Take the limit in which infinitely many gratings empty space→
Do nuclei really behave classically ?
Do nuclei really behave classically ?
Do nuclei really behave classically ?
Do nuclei really behave classically ?
Example: proton transfer in malonaldehyde
only one quantum proton
Tuckerman and Marx, Phys. Rev. Lett. (2001)
Example: proton transfer in water
Classical H5O2+ Quantum H5O2
+
Marx et al. Nature (1997)
Proton inWaterHydroxyl (Ice) Overlayers on Metal Surfaces
Li et al., Phys. Rev. Lett. (2010)
T = 160 K
Proton inWaterHydroxyl (Ice) Overlayers on Metal Surfaces
Li et al., Phys. Rev. Lett. (2010)
The density matrix: definition
Ensemble of states:
Ensemble average:
Let's define:
ρ is hermitian: real eigenvalues
The density matrix: time evolution and equilibrium
At equilibrium:
ρ can be expressed as pure function of H and diagonalized simultaneously with H
The density matrix: canonical ensemble
Canonical ensemble:
Canonical density matrix: Path integral formulation
One particle one dimension:
K and Φ do not commute, thus, Trotter decomposition:
Canonical density matrix: Path integral formulation
Evaluation of:
Canonical density matrix: Path integral formulation
Matrix elements of in space coordinates:
acts on the eigenstates from the left:
Canonical density matrix: Path integral formulation
Where it was used:
It can be Monte Carlo sampled, but for MD we need momenta!
Path integral isomorphism
Introducing a chain frequency and an effective potential:
Path integral molecular dynamics
Fictitious momenta: introducing a set of P Gaussian integrals
: convenient parameter
Gaussian integrals known: (adjust prefactor )
Does it work?
i. change of integration variables: uncouple harmonic term;ii. multiple time scale numerical integration algorithm; inherent time scales of the harmonic and external potential terms treated with appropriate time steps;iii. a highly efficient canonical MD method that rigorously generates a canonical phase space distribution (thermostat).
Many body path integral
● Each bead evolves at temperature P∙T
● P determined by how “quantum” the problem is
● Mapping converges to give the exact quantum mechanical result as P is increased
Path integrals: expectation values
Same formula, but more efficient to evaluate:
eigenvalue of O acting on a coordinate eigenstate
Path integrals: expectation values
Discretized (and finite) evaluation:
So that:
Estimators for thermodynamic quantities (derivatives of Z):
Alternative / complementary approach:Colored thermostats
Stochastic velocity rescaling thermostat
Langevin dynamics
Langevin dynamics
Colored noise thermostat
Example: liquid water
PIMD
classical
Colored thermostat