To provide theoretical insight into the cage structure and metal position of Sc-2@C-66, density functional calculations are carried out. The endohedral structure determined by the maximum-entropy-method (MEM)/Rietveld analysis of synchrotron X-ray powder diffraction data does not correspond to an energy minimum. Instead, a much more stable C-2v structure is found, which satisfies the observed C-13 NMR spectrum. Two Sc atoms are highly stabilized in the non-classical C-2v fullerene cage containing two pairs of threefold fused pentagons. (C) 2002 Elsevier Science B.V. All rights reserved.