def __init__(self, scaling, coordinates=None):
if coordinates is not None:<EOL><INDENT>self.update_coordinates(coordinates)<EOL><DEDENT>self.scaling = scaling<EOL>self.scaling.ravel()[::len(self.scaling)+<NUM_LIT:1>] = <NUM_LIT:0><EOL>
Initialize a pair potential object

           Arguments:
             scaling  --  symmetric NxN array with pairwise scaling factors.
                          When an element is set to zero, it will be excluded.

           Optional argument:
             coordinates  --  the initial Cartesian coordinates of the system,
                              which can be updated with the update_coordinates
                              method