Note that f2 ≥ 0 because it is a sum of squared coefficients of the k 1 r terms of A0,y and Ai,x. Since the f2 parameter appears in the off-diagonal components of the 2 × 2 Hamiltonian in Eq. ( ), the parameter represents the spin mixing effects enabled by SOC. In particular, when bands are inverted and cross along a nodal line in the absence of SOC, f2 = 0. If SOC is present in the system, there are two possibilities. One possibility is that additional crystallographic symmetries forbids a band gap from opening along the nodal line, such as the three-fold rotational symmetry in Na3Bi. Otherwise, a band gap opening due to SOC as in Figure can be captured with f2 > 0.