Analysis of hyperpolarizability density is helpful to understand the nature of molecular hyperpolarizability . The local contribution functions of the x, y, and z components of the second hyperpolarizability density for the carbon chains H-(C≡C) n -H (n = 3-9) are rendered in Figs. , and 2, respectively. It can be seen that the isosurfaces of positive and negative -z (3) zzz (𝑟 ⃗) are both distributed on the sides of the z axis with the molecular center as the center of symmetry, while the positive region is significantly broader than the negative part. That is the reason why all carbon chains have notably positive 𝛾 zzzz (∞) values. Notably, the isovalue used for presenting the x and y components of second hyperpolarizability density in Figs. and S3 is far less than that for the z component in Fig. , which is in line with the fact that the 𝛾 xxxx (∞) and 𝛾 yyyy (∞) are much smaller than the 𝛾 zzzz (∞). α iso (∞) = -10.221 + 17.358n 1.503 (1)