2d simulation

I want to create circular regions in a two-dimensional simulation and in a rectangular simulation box and place atoms in those circular regions, how can I do this?

How about using a cylinder region?

I want to create nanoparticles and i want my nanoparticles to be circular. How can I create circular nanoparticles in 2d simulation?

If you are discarding advice been given, you have to explain why?

Your description is quite vague. Can you be more specific?