The electronic and molecular structure of C4: Multireference configuration-interaction calculations

Multireference configuration-interaction methods have been used to calculate equilibrium geometries for different C4 structures, using large basis sets of atomic natural orbitals. The ground state of C4 is found to be linear, cumulene-like with a 3∑−g electronic state and with an energy 4.1 kcal/mol below the rhombic structure. We show that the choice of basis set plays a crucial role in the determination of the ground-state conformation.