def __init__(self, scaling, As, Bs, coordinates=None):
PairFF.__init__(self, scaling, coordinates)<EOL>self.As = As<EOL>self.Bs = Bs<EOL>
Initialize a ExpRepFF

           Arguments:
             scaling  --  symmetric NxN array with pairwise scaling factors.
                          When an element is set to zero, it will be excluded.
             As  --  A matrix with pre-exponential factors
             Bs  --  A matrix with exponents

           The repulsion has the form A*exp(-B*r)

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