Background: If you pack $n^2$ circles in a square, for 1, 4, 9, 16, 25, and 36 circles, the densest packing is with the circles stacked in an $n\times n$ grid. But for 49 circles the densest packing becomes irregular - for large $n^2$ it approaches a hexagonal packing (see http://hydra.nat.uni-magdeburg.de/packing/csq/csq.html#overview).
Question So my question is when (I am assuming it does at some point), if packing $n^3$ spheres in a cube, does regular cubic packing become less efficient than some irregular approximation of fcc or hexagonal packing?