Ab initio Study of the Energy Difference Between the Cyclic and Linear Forms of the C6 Molecule.

The energy difference between the three lowest‐lying isomers of C6, the linear 3Σ−g state, and the two ring forms, the benzene structure (1A1g) possessing D6h symmetry and a distorted cyclic form (1A1’, D3h symmetry) have been calculated using various ab initio methods. Variational methods such as multireference configuration interaction (MR‐CI) and complete active space second order perturbation treatment (CASPT2) have been applied, as well as perturbational treatments and coupled cluster calculations (CCD). The correlation of all valence shell electrons is found to be important for a balanced description of the isomers of C6. Methods which do not account for higher‐order effects appropriately proved to be unsuitable for calculating the energy difference correctly. The results from multireference configuration interaction methods show that the isomers are close in energy with the cyclic forms somewhat lower than the linear form. The ring form possessing D3h symmetry (1A1’) is found to be the lowest‐lying...