We report a detailed theor. anal. of the structures of various cryst. phases of pure and hydrogenated carbon. Using first-principles d. functional theory calcns., we propose possible structures for the exptl. obsd. phases of carbon nanocrystals through incorporation of interstitial hydrogen atoms, at proper concns., into cubic carbon lattices. The results of our anal. provide possible interpretations for the structures of i-carbon, a cubic phase with lattice parameter a = 4.25 Å, a body-centered cubic carbon phase with a = 3.1 Å, and n-diamond, a face-centered cubic phase with a = 3.57 Å.