Remarkable basis set effects on the electronic states and spin orbital polarization of a (5, 5) carbon nanotube containing a single or double carbon adatoms are revealed in density functional theoretical calculations. Although the selected five double zeta basis sets and two triple zeta basis sets all predict the same ground state (triplet) for the single carbon defect system, they predict a triplet state using polarization functions and a quintet state without using polarization function for the double carbon defect system, due to the magnification of the 2s orbitals' and reduction of 2p orbitals' contributions by the polarization functions. (C) 2013 Elsevier B.V. All rights reserved.