The vibrational frequencies of fullerenes from an updated QCFF/PI Hamiltonian

The vibrational force fields of the two most abundant fullerenes, C-60 and C-70), are computed with a version of the quantum consistent force field for pi electrons (QCFF/PI) Hamiltonian in which the parameter used to mimic the a-electron potential due to CCC bond angles of conjugated carbon frameworks is modified. It is shown that the single parameter change leads to CCC force constants close to the scaled ab initio force field values and reduces considerably the discrepancies between computed and observed vibrational frequencies of unsaturated hydrocarbons. Applied to buckminsterfullerene, it reduces the average deviation of computed vibrational frequencies from 55 cm(-1) to 31 cm(-1) and brings the accuracy of QCFF/PI close to the best first principles calculations available to date for C-60,C- at a fraction of the cost. The new set of computed C-70 vibrational frequencies is presented and an updated assignment of the experimentally available vibrational data is proposed.