Most meaningful for bond critical points,
        can be physically interpreted as e.g. degree
        of pi-bonding in organic molecules. Consult
        literature for more information.
        :return:

def ellipticity(self):
        '''
        Most meaningful for bond critical points,
        can be physically interpreted as e.g. degree
        of pi-bonding in organic molecules. Consult
        literature for more information.
        :return:
        '''
        eig = np.linalg.eig(self.field_hessian)
        eig.sort()
        return eig[0]/eig[1] - 1