The electronic and atomic structure of Sb4 and Sb8 clusters is studied using the ab initio molecular dynamics method in the local density approximation. While for Sb4 a regular tetrahedron is about 2.0 eV lower in energy than a bent rhombus, for Sb8 two weakly interacting tetrahedra lie only 0.117 eV lower in energy than a bent rhombus interacting with a stretched tetrahedron, indicating the importance of the bent rhombus structure for larger clusters. The binding energy of Sb8 relative to two noninteracting tetrahedra is about 0.5 eV. These results are in excellent agreement with the experimental data. {\textcopyright} 1993 The American Physical Society.