The energetics of small zigzag carbon nanotubes at high temperature regime (T = 2500 K) is investigated via k-space tight-binding molecular dynamics (TBMD) simulations. It is found that the n dangling atoms at the end of a zigzag tube with 5 < n < 12 can automatically transform into an (n - 1)-member ring at the top, accompanied by the release of a C, dimer and the formation of (n - 1) pentagons and a hexagon-pentagon pair involving up to five layers of atoms. For tubes larger than (11, 0) or smaller than (6, 0), it is found that energetically it is more favorable for the tube to remain open. (C) 2002 Elsevier Science B.V. All rights reserved.