SAC and SAC-CI calculations of excitation and circular dichroism spectra of straight-chain and cyclic dichalcogens

Accurate quantum-chemical calculations of the excitation energies and the rotatory strengths of dichalcogens R-Ch-Ch-R (Ch = S, Se, Te) are performed with the symmetry adapted cluster (SAC) and SAC-configuration interaction (CI) methods. The first and second lowest bands have depended on the R-Ch-Ch-R dihedral angle and the calculated trends have agreed with two empirical rules, the [C.sub.2] rule and the quadrant rule.