Hi guys, I've been exploring the idea of changing radius to be correct for periodic boxes. Right now it is incorrect; each component (x,y,z) should not have a distance greater than 0.5 * domain_size. The easiest way to do this would be: rx = min( abs( x - center_x ), abs( x - center_x - domain_x) ) I think the best way to do this is to set up a NumPy ufunc http://docs.scipy.org/doc/numpy/user/c-info.beyond-basics.html#creating-a-ne... that accepts three arrays, along with the center and the domain width, and then returns the radius. What do you all think? Alternatively, I'm thinking maybe just a regular function that gets the arrays and returns one would be better; the ufunc machinery is a bit complicated and I might get confused. Once I come up with it, will somebody be able to look over my work? -Matt