Undoubtedly, the NLO properties of polyynes R 1 -(C≡C) n -R 2 will be greatly affected by the terminal groups, and H-(C≡C) n -H is the closest species to the hypothetical carbon allotrope carbyne. In the work reported herein, we choose a series of H-capped carbon chains, H-(C≡C) n -H (n = 3-9, 15), as models for theoretical studies to examine the variation of their optical nonlinearities with the length of the chain.