Geometries and electronic structures of the B12 cluster have been investigated using a Car-Parrinello ab initia molecular dynamics simulation. The icosahedral structure, which is commonly found in bulk boron compounds, is locally stable, but has a few dangling bonds. On melting and annealing, this structure rearranges to a more open geometry. The new structure has a significantly lower energy despite a lower coordination. The bonds are stronger and there are no dangling bonds. © 1991 American Institute of Physics.