New convex and spherical structures of bare boron clusters

New stable structures of bare boron clusters can easily be obtained and constructed with the help of an ''Aufbau Principle'' suggested by a systematic ab initio HF-SCF and direct CI study. It is concluded that boron cluster formation can be established by elemental units of pentagonal and hexagonal pyramids. New convex and small spherical clusters different from the classical known forms of boron crystal structures are obtained by a combination of both basic units. Convex structures simulate boron surfaces which can be considered as segments of open or closed spheres. Both convex clusters B-16 and B-46 have energies close to those of their conjugate quasi-planar clusters, which are relatively stable and can be considered to act as a calibration mark. The closed spherical clusters B-12, B-22, B-32, and B-42 are less stable than the corresponding conjugated quasi-planar structures. As a consequence, highly stable spherical boron clusters can systematically be predicted when their conjugate quasi-planar clusters are determined and energies are compared. (C) 1997 Academic Press.