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Ab initio Molecular DynamicsBornOppenheimer and beyond
Reminder, reliability of MD
MD trajectories are chaotic (exponential divergence with respect to initial conditions), BUT....
With a good integrator (e.g., Verlet or velocity Verlet) the discrete trajectory, with the same initial and final point as the exact analytical one, can be made infinitely close to the latter (shadow theorem/hypothesis).So: the dynamical information are reliable.What about the ensemble averages?
The ergodic theorem/hypothesis tells us that time and ensemble averages are equivalent (at the limit of infinite sampling time). Not true for all systems, though (indeed called nonergodic).
Ab initio MD, general formalism
Schrödringer equation for a system of N nuclei and n electrons:
acts on nuclei acts on electrons
Ab initio MD, general formalism
Suppose the solutions of the time independent electronic Schrödinger equation are known
: correction to the adiabatic eigenvalue of
Ab initio MD, BornOppenheimer approximation
If all are negligble:
Adiabatic approximation (BornOppenheimer):
Valid in many cases, but, notably, not for electron transfer, photoisomerisation, ...
Ab initio MD, semiclassical approach
adiabatic eigenfunctions
classical trajectory
: probability of finding the system in adiabatic state i at time t
Ab initio MD, validity of the BO approximation
Massay parameter
characteristic length
velocity of the nuclei
time scale of electronic motion
Ab initio MD, semiclassical BO approximation
The system, when prepared in one adiabatic state i stays there forever
Nuclei move according to Newton's equations:
Non adiabatic: mean field (Ehrenfest) dynamics
The atoms evolve on an effective potential representing an average over the adiabatic states weighted by their state populations
Forces: HellmannFeynman theorem:
nonadiabatic coupling vectors
Ehrenfest dynamics
A system that was initially prepared in a pure adiabatic state will be in a mixed state when leaving the region of strong nonadiabatic coupling. The pure adiabatic character of the wavefunction cannot be recovered even in the asymptotic regions of configuration space.
The total wavefunction may contain significant contributions from adiabatic states that are energetically inaccessible.
Ehrnefest dynamics , violation of microscopic reversibility
Surface hopping (fewest switches)
Supposing:
At a later time:
Let there be N trajectories:
Surface hopping (fewest switches)
Transition selected by using random numbers
Transition from i to k invoked if :
Uniform random numberBetween 0 and 1
Surface hopping (fewest switches)
Sum of the transition
probabilities of the first k states
Photoisomerization of formaldimine
Photoisomerization of formaldimine
R P→ R R→
E.g., dipolar molecules:
dipoledipole tensor
permanent + induced dipole moment
Induced dipoles follow nuclei adiabatically and is always at its minimum:
Suppose a fluid of polarizable molecules
CarParrinello MD, a classical system
Iterative solution of (3N) linear equations?
CarParrinello MD, a classical system
Extended Lagrangian:
Equation of motion for dipoles:
Role of M?
Adiabadicity: two temperatures?
units?
CarParrinello MD, the idea
Starting from minimized KohnSham orbitals, define orbital velocities and kinetic energy:
Lagrange multipliers
Equations of motion:Functional derivative. In practice wrt the coefficients of the chosen basis set
Initial forces:
Constraint?
Initial conditions:
Update velocities (Δt/2)
Update positions (Δt)
CarParrinello MD, the algorithm
Iterative solution:
Starting from:
CarParrinello MD, the algorithm
1
4
5
6
2
3
Velocities: Positions:
Forces update:
CarParrinello MD, the algorithm (summary)
Molecular dynamics:summary of extended systems
and applications of BornOppenheimer MD
Reminder, reliability of MD
MD trajectories are chaotic (exponential divergence with respect to initial conditions), BUT....
With a good integrator (e.g., Verlet or velocity Verlet) the discrete trajectory, with the same initial and final point as the exact analytical one, can be made infinitely close to the latter (shadow theorem/hypothesis).So: the dynamical information are reliable.What about the ensemble averages?
The ergodic theorem/hypothesis tells us that time and ensemble averages are equivalent (at the limit of infinite sampling time). Not true for all systems, though (indeed called nonergodic).
Summary of extended systems: NVT (Nosé)
Lagrangian:
Conjugated momenta:
Hamiltonian
Eq. of motion:
Summary of extended systems: NPH (Andersen)
Summary of extended systems: NPH (ParrinelloRahman)
Summary of extended systems: onthefly optimization
CarParrinello MD
Classical example: polarizable fluid
Galli et al. Science 1990
300 fs
Application of plain MD: nature of liquid C
64 C atoms
5000 K
Application of plain MD: nature of liquid C
Galli et al. Phys. Rev. Lett. 1989
Application of plain MD: dissociative adsorption
Phys. Rev. Lett. 1993
Initial @ Collision Final (400fs)
Trajectory 1
Trajectory 2
Trajectory 3
Trajectory 4
Trajectory 5
Kinetic energy:1 eV
Kirchner and Hutter, J. Chem. Phys. 2004
Application of plain MD: solvation of DMSO in water
gas phaseDMSO
DMSOWater
32 water molecules + 1 DMSO@ 320 K