Hi Shahab, Adding to Luca's reply you probably want to look into the `ParticleHandler` class, in particular the `particles_in_cell` function that returns you all particles in a given cell (https://dealii.org/developer/doxygen/deal.II/classParticles_1_1ParticleHandler.html#acaf1232ffce0746baa64122a5c65822e). By looping through the cell's neighbors you can also find all particles in neighbor cells.
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