Accelerated Molecular Dynamics Methods


Tim
Germann  tcg@lanl.gov  Los Alamos
National Laboratory 
Outline of Lecture
 Background
 Rare event examples: epitaxial growth, radiation damage
 e.g. plutonium aging due to selfirradiation, ...
 Pitfalls of KMC: concerted multiatom mechanisms, unconventional
crystal structures or grain boundary diffusion, ...
 One possibility: HenkelmanJonsson saddlepoint enumeration
+ KMC Seek MD acceleration technique(s) w/o biasing the dynamics
 Hyperdynamics
 Basic idea: boost PES in wells w/o affecting saddle points
Mathematical justification (assuming TST) & definition
of an accelerated time
 Demonstration on model system(s) using simple boost functions
 More sophisticated boost functions
 Application to more complex systems
 Unsolved problems: extension to systems with low barriers
(or a range of barriers, e.g. glasses, liquids, proteins)
 Related approaches: temperature accelerated dynamics
 Basic idea: increase simulation temperature to find neighboring
saddles, but reject transitions until lowtemperature path
can be accepted
 Mathematical justification (assuming harmonic TST)
 Demonstration on model system(s)
 Related approaches: parallel replica dynamics
 Basic idea: independent realization of system on each processor
to explore phase space more rapidly; first processor to find
escape pathway is accepted
 Mathematical justification (assuming infrequent
events)
 Demonstration on model system(s)
 Brownian Monte Carlo
 Combining II and IV > parallel replica hyperdynamics
 Multiplicative boost achieved
 Example: epitaxial growth of Cu/Cu(100) on 1000 procs >
0.3 s
 Outlook & Prospects
 Lab
 Will focus on *'d topics; vary boost function, system size
and complexity (e.g. single adatom diffusion vs. islands with
both low and high barriers) to demonstrate where hyperMD works
well and where it runs into difficulties.


