Objective

In this tutorial, you will use the Modified Embedded Atom Method (MEAM) in Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS) to acquire the dislocation mobility drag coefficient for your material.

You can obtain LAMMPS and find the user manual here. Visit the LAMMPS tutorials page for a wide range of examples. LAMMPS video tutorials - Intro, Input file Part 1, Input file Part 2, Molecular Dynamics Theory and Examples.

Environment Setup

LAMMPS is an open source code and can be downloaded here. The source code can be easily compiled, or binary distributions are available for easy installation.

In addition to the software, you will need an input file, an atom position file, a post-processing script, and two files for your atomistic potential.

• The input file for this simulation is DislocationVelocity.
• The atom position file will be created by Atomistic Dislocation Generation.
• The post-processing script uses Ovito's scripting and analysis capabilities: Single Defect Velocity in Ovitos.

Calculating Dislocation Velocity

Walkthrough

Video tutorial: Part 1 and Part 2

Step 1

Generate the atomic structure using Atomistic Dislocation Generation. Once you have downloaded and compiled the fortran routine, as directed, simply run it at the command line as explained on the script page. For this calculation, you want to make an edge dislocation in a PAD geometry. It will create the required data file as atoms.*.edge.pad, where * is either fcc or bcc, depending on your material. You will need to copy the data file to the directory where you will run the MD simulation.

Step 2

Edit the LAMMPS input file for your material, applied stress, and desired temperature. These can all be set by variables at the top of the input file.

# Variable definitions
variable initTemp equal 300.          	# desired temperature
variable sigma equal 15000.    		# applied stress in bar
variable material string Ta		# material symbol
variable atom_file string atoms.bcc.edge.pad # the configuration was generated by SG with the preprocessor dislocation.f90

initTemp will determine the temperature of the system.

sigma will determine the applied shear stress in bar.

material will determine the name of your material used in your MEAM potential files. It expects to find a library file, *.library.meam, and a parameter file, *.meam where the * represents the material symbol.

atom_file is the name of the data file that contains the initial atom positions.

Step 3

LAMMPS will create several output files from the simulation. You can load each of them in Ovito to see what each includes. You will run the post-processing script, Single Defect Velocity in Ovitos, on the dump.shear.unwrap file, following the directions on the script page.

It will output a time versus position file which you can use to calculate the velocity of the dislocation.