Ab initio study of C20 isomers: geometry and vibrational frequencies

Geometry optimizations are performed for three isomers of C20, namely the ring, bowl (corannulene-like), and cage (fullerene-like) configurations, using both the local density functional approximation (LDA), and gradient-corrected density functional theory (BLYP). The BLYP results confirm the previous single point calculation based on Hartree-Fock geometries, which is in favor of the ring geometry, while the LDA results still indicate a reversed order of energy, thus favoring the cage geometry. Our calculated LDA vibrational spectra of the ring geometry show significant differences with those of HF in the widths of two band gaps. In addition, our Mّller-Plesset second-order perturbation theory (MP2) single point calculations using GAMESS show preference towards the bowl structure, which differs from previous similar calculations.