What goes around comes around
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    * **Topic:** periodic boundary conditions 
    * **Task:** 
        1. Implement the function that calculates the squared distance between
           particles, taking into account the periodicity of the system.
        2. Run the simulation. The small particle should bounce off the larger ones. Does it?
        3. Draw the system so that a few periodic images are also visible.
    * **Template:** `pbc.py <http://users.utu.fi/tejohy/teaching/simu/templates/pbc.py>`_
    * **Data:** `supercell.txt <http://users.utu.fi/tejohy/teaching/simu/templates/supercell.txt>`_
    * **Further reading:** https://en.wikipedia.org/wiki/Periodic_boundary_conditions


pbc.py
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.. automodule:: pbc
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    :undoc-members: