STRUCTURE AND STABILITY OF SOME C60 ISOMERS

We have investigated the stability of the ideal C60 molecule and of some isomers using a molecular-dynamics quenching technique. The atomic forces used for the quench algorithm are obtained from a minimal-orbital sp-basis tight-binding calculation with local charge neutrality placed on each site. The isomers are derived from the ideal structure by introducing one or more tiling faults. As expected, the most favorable structure is found to be the ideal C60 molecule. The introduction of one tiling fault leads to a change in the cohesive energy of about 0.02 eV per atom (1.3 eV per molecule) and the corresponding value for the introduction of two faults, whether isolated or coupled, is found to be about twice this value. These changes are extremely small in comparison with the absolute value of the cohesive energy, with the fault energy equal to about one-fifth of the cohesive energy per atom of ideal C60 (calculated to be 6.94 eV). In addition, we find that an isomer containing a seven-membered ring has a fault energy of about 2.7 eV. We conclude that the existence of these isomers is probable on energetic grounds although the considerable deformation and consequent stress in the isomer containing a seven-membered ring leads us to suggest that it would not be observed experimentally.