An analysis of the crystal structure of the beta polymorph of isotactic polypropylene (beta-iPP) is carried out with the methods of molecular mechanics, taking into account simultaneously intra- and intermolecular interactions. Results indicate that efficient crystal packing may be achieved in a number of different, nearly isoenergetic arrangements, most of them showing the features of frustration. This fact may well be due to the low density of beta-iPP as compared to its other crystalline polymorphs. The most stable beta-iPP structure is found in space group P3(1). Up-down disordering modes, found also in a and gamma-iPP, have marginally higher energy and give rise to P3(1)21 symmetry, without superposition of methyl groups belonging to anticlined chains. This structure, involving random up-clown disordering, is thus proposed as the best average structure for individual beta-iPP domains. Mechanisms for twinning among different domains are also investigated in order to explain the observed hexagonal symmetry of electron diffraction patterns.