LDA PREDICTIONS OF C-20 ISOMERIZATIONS - NEUTRAL AND CHARGED SPECIES

The energies of isomers of C-20 including neutral and positively charg ed ring, fused ring, flake, and fullerene structures have been calcula ted within the pseudopotential local density approximation(LDA). The o bjective is to predict the relative energies of the isomers as well as to validate LDA calculations for the carbon system. For C-20, high-le vel coupled-cluster (CCSD(T)) calculations are just possible and are u sed for comparison. Our most accurate LDA calculations agree with prio r calculations quantitatively and remarkably well with extrapolated CC SD(T) calculations. However, various approximations can qualitatively affect the results. For example softening the pseudopotential for carb on can cause the ordering of the isomer stability to reverse even thou gh good agreement with smaller systems is retained. Similar problems c an occur if the plane-wave basis is not sufficiently large.