Erm... i dont understand, its not really the point how fast sqrt() is , the main thing is your doing an extra calculation that is completly redundant,
If you have 10,000 atoms then 10,000*10,000 sqrt()'s has an effect on performance,
It must be better to do just one $cutoff_dist**2?
However i do agree that the any improovments to the brute force method would be better,
Cheers , monkey_boy

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