PPP Model Parameters for Fullerenes

It is often assumed that resonance integrals are proportional to the overlap between the atomic orbitals which formally underlie the pi Hamiltonian [58]. If these are assumed to be ordinary 2p atomic orbitals for carbon, the distance dependence of the overlap can be calculated analytically, and the distance dependence of the t parameters is often taken to be of this form, as for example in Extended Hiickel Theory [59]. But there is no need to make this assumption since the parameters in the pi Hamiltonian should more properly be thought of as rescaled effective integrals, and there is evidence that the model performs better if the t values are allowed to vary more rapidly with distance. Accordingly, we have adopted the form 

On the other hand the Ih symmetry imposes no constraint on the lengths of the 6-6 and 6-5 bonds, other than that all 6-6 bonds must be identical and all 6-5 bonds must be identical. There are thus just two geometric degrees of freedom, and both can be represented by stretching force constants just as in planar hydrocarbons. 

Correlated calculations using SDCI plus generalized Davidson or MP2 corrections as described for the Hubbard model were first performed for buckminsterfiillerene using a standard literature set of PPP parameters [65], namely to = -2.6 ev at the benzene reference bond length To, y0 = 11.13 ev, and the Ohno form for the two center y s. While to is more commonly chosen as 

---