# How to model a crack at an angle?

Dear LAMMPS Users,
I am familiar with modeling a crack along the x, y, or z direction by grouping the atoms on either side of the crack and excluding the interatomic forces between the two groups of atoms. But now I want to have a crack at an angle with respect to x-direction for example. Can you please help me?
Sincerely,
Melika Vokhshoori

Why can’t you use the exact same method?

Dear Dr. Kohlmeyer,

I have difficulty forming a region at an angle. In order to delete the interatomic forces between two atom groups to form a crack at an angle, I need to define these groups of atoms at an angle which I don’t know how to do and I haven’t found any input examples which have done so.

Regards,

Melika

You cannot expect examples for all circumstances. For advanced cases you need to derive solutions from the descriptions of features in the manual.

Regions can be rotated, you can build more complex regions through using intersections and unions (e.g. intersect box and plane) and you can create groups based on atom style variables which allow for arbitrary complex regions.

Thank you so much for your help. I try to make it work.
Regards,
Melika

I am sharing a snippet to create an hourglass-shaped region with open ends:

``````# Create a fancy region to funnel molecules.
variable xc equal \$x/2
variable yc equal \$y/2
variable y4 equal \$y/8
region c1 cone x \${yc} \${yc} \${yc} \${y4} 0     \${xc} open 1 open 2
region c2 cone x \${yc} \${yc} \${y4} \${yc} \${xc} \$x    open 1 open 2
region funnel union 2 c1 c2
fix wall all wall/region funnel harmonic 20.0 0.0 4.0
``````

Also, you can build LAMMPS as a library and call it inside Ovito to have an interactive rendering of input scripts. This is very handy for debugging complex simulations and having graphical feedback. You find the instructions and video explanations in the 2021 User meeting material.
Let us know if you succeed in modelling the crack, possibly sharing your script with the community.

Thank you so much for your help. I will surely take your suggestions and guidance into consideration and hopefully, I can succeed. I will share my script once I get reasonable results.
Sincerely,
Melika Vokhshoori