def __init__(self, force_constant, indexes, icfn):
self.force_constant = force_constant<EOL>self.indexes = indexes<EOL>self.icfn = icfn<EOL>
Arguments:
 | ``force_constant`` -- The force constant in atomic units.
 | ``indexes`` -- The indexes of the atoms in the internal
                  coordinate. The order must be the same as the order
                  of the mandatory arguments of icfn.
 | ``icfn`` -- a function from molmod.ic that can compute the
               internal coordinate and its derivatives.