def hessian(self, coordinates):
<EOL>N3 = coordinates.size<EOL>hessian = numpy.zeros((N3,N3), float)<EOL>for term in self.terms:<EOL><INDENT>term.add_to_hessian(coordinates, hessian)<EOL><DEDENT>return hessian<EOL>
Compute the force-field Hessian for the given coordinates.

           Argument:
            | ``coordinates`` -- A numpy array with the Cartesian atom
                                 coordinates, with shape (N,3).

           Returns:
            | ``hessian`` -- A numpy array with the Hessian, with shape (3*N,
                             3*N).