Cn(n=2-4): current status

The major aspects of the C-2, C-3 and C-4 elemental carbon clusters are surveyed. For C-2, a brief analysis of its current status is presented. Regarding C-3, the most recent results obtained in our group are reviewed with emphasis on modelling its potential energy surface which is particularly complicated due to the presence of multiple conical intersections. As for C-4, the most stable isomeric forms of both triplet and singlet spin states and their possible interconversion pathways are examined afresh by means of accurate ab initio calculations. The main strategies for modelling the ground triplet C-4 potential are also discussed. Starting from a truncated cluster expansion and a previously reported DMBE form for C-3, an approximate four-body term is calibrated from the ab initio energies. The final six-dimensional global DMBE form so obtained reproduces all known topographical aspects while providing an accurate description of the C-4 linear-rhombic isomerization pathway. It is therefore commended for both spectroscopic and reaction dynamics studies. This article is part of the theme issue 'Modern theoretical chemistry'.