Twin formation in silicon growth from the melt is examined by molecular dynamics (MD) simulations. For a moderate undercooling of 25 K. we find that twins do not nucleate on (111) microfacets in the perfect crystal, but exclusively occur in the vicinity of grain boundaries. Only at an undercooling of 150 K, we observe the formation of metastable twin bounded loops with incoherent interfaces to the matrix consisting of coherency and anticoherency dislocations. In conclusion, the nucleation of stable twins in silicon growth requires the presence of a grain boundary or more general of a three-phase boundary, but is unlikely to occur on ideal (111) facets because of the excess energy of the interfacial area between matrix and twinned crystal. (C) 2009 Elsevier B.V. All rights reserved.