Atomic structure and lattice vibrations of Pandey's π bonded chain model of the clean diamond (111) surface are studied based on the density functional theory. In order to check the validity of the calculations, we examine both local density approximation and the generalized gradient approximation. Two repeated slab approximations for simulation of surfaces are also examined: the two identical surfaces are attached to the slab at the top and the bottom in one approximation and one side of the slab is terminated by hydrogen in the other approximation. All the present calculations based on a variety of the approximations indicate that the π bonded chain is undimerized and unbuckled and, therefore, the band gap does not open. These results are in sharp contrast with a recent experimental suggestion that the π-bonded chain is substantially buckled. Instead of the dimerization and buckling, we found anomalous softening of the π bond stretching mode at the Γ point. The recently proposed π bonded trimer model is found to be highly unstable, while the chain model is found to be the most stable.