Surface reconstructions of unterminated 4H-SiC(0001) surfaces have been investigated theoretically using the first principal density functional theory. A (2 x 1) reconstruction was found for the Si(0001) surface, whereas the C(000 (1) over bar) surface retained its initial (1 x 1) structure. The downward relaxation was, on the other hand, much larger for the C surface than for the Si surface. The effects of adsorption of C2H2 (or Si) on the two surfaces were also studied. The adsorbates were then observed to bond strongly to the surfaces of Si(0001) and C(0001), respectively. They also influenced the surfaces in the direction of bulk parameters.