We study the electronic states of giant single-shell and the recently discovered nested multishell carbon fullerenes within the tight-binding approximation. We use two different approaches, one based on iterations and the other on symmetry, to obtain the π-state energy spectra of large fullerene cages: C240, C540, C960, C1500, C2160 and C2940. Our iteration technique reduces the dimensionality of the problem by more than one order of magnitude (factors of ∼ 12 and 20), while the symmetry-based approach reduces it by a factor of 10. We also find formulae for the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) energies of C 60·n 2 fullerenes as a function of n, demonstrating a tendency towards metallic regime for increasing n. For multi-shell fullerenes, we analytically obtain the eigenvalues of the intershell interaction.PACS numbers: 31.15.+q, 02.90.+pIntroduction.-The discovery of a new simple technique 1 for the production in bulk quantities of fullerenes has triggered intensive research on these new all-carbon molecules and the search for other novel forms of carbon. Subsequently, a new type of carbon structure composed of multilayered needle-like tubes has been discovered by high-resolution transmission electron microscopy 2 . Quite recently, giant nested shells of onion-like fullerenes (also called hyperfullerenes 3 ) have been synthesized by intense electron-beam irradiation 4 . Essentially they consist of a composition of concentric spherical fullerene cages. The innermost cage is a C 60 molecule which is encapsulated by giant fullerenes C 240 , C 540 , · · ·, one following the other 4,5 . The interlayer spacing coincides with that for bulk graphite (3.34Å). A recent investigation 6 on the stability of these brand-new diverse forms of carbon shows that multi-sh Specifically, a very recent letter 7 shows that a stability transition from single to multilayer fullerenes occurs when the number of atoms exceeds ∼ 6000. Moreover, the concentric carbon onion structure containing no dangling bonds provides a challenge to graphite (comprising flat sheets of carbon hexagons) for the most stable form of pure carbon.The structures and energies for several carbon "onions" have recently been carried out by using realistic atomic potentials 7 and molecular mechanics calculations 8 . As yet there has been little spectroscopic information available regarding the electronic properties of the multi-shell fullerenes, besides total energy calculations. The exact diagonalization of a local orbital matrix scales as N 3 (for a N −dimensional matrix). The orthonormalization step in plane-wave methods also scales as N 3 . These two examples illustrate typical bottlenecks encountered when studying one of the central thrusts of condensed matter theory: to compute the energetics of very large systems. Different suggestions on how to minimize this formidable problem have recently attracted a lot of attention (see, e.g., Ref. 9). Here, we explore different ways to reduce this difficulty by using two very dissim...