def add_to_hessian(self, coordinates, hessian):
<EOL>q, g = self.icfn(coordinates[list(self.indexes)], <NUM_LIT:1>)<EOL>for ja, ia in enumerate(self.indexes):<EOL><INDENT>for jb, ib in enumerate(self.indexes):<EOL><INDENT>contrib = <NUM_LIT:2>*self.force_constant*numpy.outer(g[ja], g[jb])<EOL>hessian[<NUM_LIT:3>*ia:<NUM_LIT:3>*ia+<NUM_LIT:3>, <NUM_LIT:3>*ib:<NUM_LIT:3>*ib+<NUM_LIT:3>] += contrib<EOL><DEDENT><DEDENT>
Add the contributions of this energy term to the Hessian

           Arguments:
            | ``coordinates`` -- A numpy array with 3N Cartesian coordinates.
            | ``hessian`` -- A matrix for the full Hessian to which this energy
                             term has to add its contribution.