Electronic correlation energy in linear and cyclic carbon tetramers

Theoretical studies have predicted the ground electronic states of the linear and cyclic (rhombic) forms of the carbon tetramer to be nearly isoenergetic. Previous theoretical studies have not explicitly considered the effect of higher than triple excitations on the ground state electronic correlation energy. In the set of calculations presented in this Letter, configuration interaction (CI) calculations are performed and no restriction on excitation level is imposed. The effect of quadruple and higher excitations freely enter the calculations. No previous study has been conclusive in determining the relative stability of the two carbon tetramer isomers. The CI calculations presented predict the rhombic structure to be energetically more stable than the linear form for all levels of approximation considered. The effect of inclusion and exclusion of triple and quadruple excitations on the ground state energy of the carbon tetramer is discussed.