We present a novel method to calculate energies of nanotube caps with different levels of accuracy and a comprehensive study of its application to the IPR caps of the (10,0) carbon nanotube. The two most stable caps for (10,0) have 42 atoms, an energy of 8.7 eV, and correspond to sections of the third most abundant fullerene, C-84. These caps are isoenergetic with a chemically unstable cap with 40 carbon atoms related to a C-80 isomer that is also chemically unstable. Energies for the other caps are between 9.3 and 10 eV. A method to calculate cap energetics with fullerenes with an error less than 3% is also presented.