NEEP 541 Molecular Dynamics Fall 2002 Jake Blanchard

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

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

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

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”

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

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

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

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)

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

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:

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

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

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

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