Subroutine calc_connect

Fast $O(N)$ connectivity calculation routine. It divides the unit cell into similarly shaped subcells, of sufficient size that sphere of radius cutoff is contained in a subcell, at least in the directions in which the unit cell is big enough. In narrow directions, the unit cell is replicated on the fly to collect neighbours that are images of the stored atoms. For very small unit cells, there is only one subcell, so the routine is equivalent to the standard $O(N^2)$ method.

this -- type(Atoms), intent(inout)

own_neighbour -- logical, optional, intent(in)

partition_atoms subroutine