QUANTUM MOLECULAR-DYNAMICS SIMULATIONS OF FULLERENES AND GRAPHITIC MICROTUBULES

We describe the results of extensive ab initio molecular dynamics calc ulations of the properties of fullerenes and microtubules. Our finite temperature quantum MD simulations for solid C60 are in excellent agre ement with NMR, photoemission and neutron scattering data. The C60 iso mer containing two pairs of adjacent five-fold rings has a binding ene rgy only 1.6 eV smaller than that of perfect C60, but the transformati on between these two structures is hindered by a 5.4 eV barrier. It th us requires high temperatures and long annealing times. High temperatu res are also needed for the transformation of the lowest energy C20 is omer, a dodecahedron, to a corannulene structure, which can be thought of as a fragment of C60. The corannulene structure is a natural precu rsor for the formation of C60. Simulations of reactions show that C2 c an insert into C58, perfect C60, and defect C60 fullerenes without an activation barrier, while C3 attaches only to their surfaces. Evaporat ive fragmentation of carbon clusters during annealing is unlikely, but atom and fragment exchange during collision favor ''locally'' most st able structures, such as C60. These results may explain the large incr ease in the abundance of C60 and C70 when carbon clusters are annealed at high density. We have also carried out calculations for paradigmat ic microtubules, both reflection-symmetric and chiral. We find that th e optimized geometries of the tubules are close to the ideal ones. It is possible to fabricate tubules with direct band gaps away from the G AMMA point by exploiting the similarities between the projected band s tructure of graphite and that of the tubule. The semiconducting tubule s can be doped n- and p-type by substitutional N and B, respectively.