Ab initio study of C-20 isomers: Geometry and vibrational frequencies

Geometry optimizations are performed for three isomers of C-20, 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 I-IF in the widths of two band gaps. In addition, our Moller-Plesset second-order perturbation theory (MP2) single point calculations using GAMESS show preference towards the bowl structure, which differs from previous similar calculations.