NEEP 541 Molecular Dynamics

1. NEEP 541Molecular Dynamics Fall 2002 Jake Blanchard

2. Outline • Molecular Dynamics • The Kalypso code • Documentation • Limitations • Potentials • Binding energy • Temperature • Electronic stopping

3. Molecular Dynamics • Codes • Moldy • Kalypso • Mdrange • Marlowe • I’ll discuss choices made by Kalypso • Other codes would make other choices

4. Kalypso • Do not trust what comes out of kalypso, or any other code, without being aware of exactly what it is doing • It contains many assumptions and approximations that might not be valid for your situation • For many cases, scaling and relative changes are more valid than absolute values of results

5. Kalypso • For studying atomic collisions in metals • Best-suited for single particle collisions with target atoms (sputtering, reflection, adsorption, implantation) • Could also model diffusion, defect stability, etc. • Cannot model semiconductors • Uses molecular dynamics • Potentials (many-body) are Sutton-Chen or “Tight-Binding”

6. Codes • Spider is for preparing input files • Kalypso is for running the simulation • Winnow is for post-processing

7. Documentation • Simulation primer • UI Guides for Spider, Kalypso, and Winnow • User Guide • Tutorial

8. Limitations • Particle energies 0.1 to 100,000 eV • Low energy requires quantum effects • Higher energy requires inelastic reactions

9. Interactions • SRIM models binary interactions with repulsive potentials • This is most useful for particle energies greater than about 10 eV • Kalypso uses more complete potentials (attractive and repulsive, many-body) • Potentials are cut-off by user (usually 1-2 nearest-neighbors)

10. Potentials • Use screened coulomb potential splined to attractive portion for short-range interactions • For low energy interactions, use many-body potentials (Sutton-Chen or tight-binding) • Fitting of attractive potential to repulsive portion is not trivial • Many-body potentials improve results near surface and for static properties

11. Potentials • Many-body potentials are similar to Lennard-Jones potentials • Attractive part of SC is square root of sum of LJ potentials • Attractive part of TB is square root of sum of Morse potentials • Morse potentials:

12. Choosing Potentials • Choose form of screened coulomb potential (choose screening radius) for both target-target and projectile-target interactions • Choose many-body potential (TB is recommended) • Details (and constants for some materials) are in Simulation Primer

13. Surface Binding Energy • Many-Body potentials require corrections near surfaces • Pair-potentials alone tend to underestimate surface binding effects • There’s no “best” approach to correcting for surface effects • Current correction in Kalypso reduces perpendicular velocity of emitted particles such that kinetic energy falls by surface binding energy

14. Thermal Vibrations • Some results will be affected by thermal oscillations of the lattice • Spider calculates mean square thermal vibrations

15. Electronic Stopping • Several models for electronic stopping are in Kalypso • Described in Simulation Primer • Effects of target temperature are corrected for