Chapter 3
Theoretical Methods and Results for Electronic Structure Calculations on Very Large Systems Carbon Clusters
Downloaded by NORTH CAROLINA STATE UNIV on December 19, 2012 | http://pubs.acs.org Publication Date: October 22, 1987 | doi: 10.1021/bk-1987-0353.ch003
Jan Almlöf and Hans Peter Luthi Department of Chemistry and Minnesota Supercomputer Institute, University of Minnesota, Minneapolis, MN 55455 Large clusters of carbon atoms have been studied with ab initio calculations using basis sets of double-ζ quality. Planar, single-sheet graphite fragments with 6 - 54 atoms were investigated, as well as the spherical "Buckminsterfullerene" C molecule. Partly hydrogenated clusters have also been considered, both graphite- and diamond-like. The largest of these, C H , was treated with 1560 basis functions, and constitutes the largest ab initio Hartee-Fock calculation performed so far. Saturation of the peripheral bonds with hydrogen is found to provide a smooth and uniform convergence of the properties with increasing cluster size. For the graphite-like clusters the convergence of properties to bulk values is much slower than for the three-dimensional complexes. 60
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Due to their scientific and technical importance, carbon clusters have long been subject to a variety of experimental(1-24) and theoretical(25-51 ) investigations. The areas of interest are of great diversity, including heterogeneous catalysis, the composition of graphite vapor, soot formation during combustion(15) and early stages of synthetic natural gas production from coal. Several small clusters have been identified spectroscopically in the atmospheres of red giant stars and in comet tails(6-9). The experimental work reported for carbon clusters ranges from the early investigations of Honig and Drowart on small clusters(1-3) to the recent studies in the groups of Smalley and Kaldor(10-16) on larger clusters with up to 200 atoms. A striking observation that lacks a satisfactory explanation is the existence of 'magic numbers', i.e. the fact that in a distribution of clusters some species with a certain number of carbon atoms are much more abundant than others. The exact clustering mechanisms are not completely understood, and, as noted by Rohlfing et al.(10), the origin of the observed distribution of clusters may depend upon instrumental factors. Accounting for this fact, however, there still seems to be a preference for clusters with certain numbers of atoms which cannot be explained solely as due to the experimental conditions. There are several methods in use for producing these clusters. Particle bombardment or laser vaporization of a graphite surface leads to direct formation of ions that can be detected by mass spectrometry. These are normally of relatively small size (n
