def METHOD_NAME(self, n, rmin, rmax,
               nNodesPerh = 2.01):
    if n > 0:
        vol = (rmax.x  - rmin.x) * (rmax.y - rmin.y) * (rmax.z - rmin.z)
        assert vol > 0.0
        dV = vol/n
        dx = dV**(1.0/3.0)
        hi = 1.0/(nNodesPerh*dx)
        Hi = SymTensor3d(hi, 0.0, 0.0,
                         0.0, hi, 0.0,
                         0.0, 0.0, hi)
        return Hi
    else:
        return SymTensor3d()