def test_fixed_sphere(self):
        # add some particles
        particle_list = [
            (1, 1, 0, espressopp.Real3D(5.0, 5.5, 5.0), 1.0, 0),
            (2, 1, 0, espressopp.Real3D(5.0, 6.5, 5.0), 1.0, 0),