  Our density functional theory calculations for Fe_{13-n}M_{n} for M = Sc, Ti,
V, Cr, Mn, Co, Ni, and Cu up to n = 4 show that the icosahedral symmetry of
Fe13, albeit minor changes in bond lengths, is robust despite doping and is
retained for all homotops of Mn, Co, Ni and Cu. Based on analysis of density of
states of the doped cluster, adsorption of carbon atom, and adhesion energies
for fragments of single walled carbon nanotubes, we propose a core-shell type
structure with a central Mn atom surrounded by Fe surface shell atoms as the
most favorable doped nanocatalyst for SWCNT nucleation and growth subject to
constraints of retention of icosahedral symmetry by the doped cluster. For
doping beyond the central atom involving the surface shell of the icosahedron,
Ni is the best candidate.
