Knowledge Bank

Electronic structure calculations were carried out for the $C4$ molecule in the linear $(3\Sigma g-)$ and rhombus (bicylic, $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Ag$) configurations. The calculations used double-zeta (DZ) and DZ plus polarization (DZP) basis sets, and were carried out at the SCF (RHF), MCSCF, single-reference CI, and multireference CI levels. The geometries of the two structures were optimized at the MCSCF level, and the resulting bond lengths are greater by $0.03-0.09\backslash AA$ than those optimized by Whiteside $etal.1$ at the SCF (UHF) level. The relative energies of the two structures are very sensitive to the basis set used and, to a lesser degree, to the type of wave function. The $D2$ basis strongly favors the linear structure, but the energies of the two structures are comparable when the DZP basis is used.