Accurate quantum-chemical calculations of the excitation energies and the rotatory strengths of dichalcogens R-Ch-Ch-R(Ch) S, Se, Te) were carried out with the symmetry adapted cluster ( SAC) and SAC-configuration interaction (CI) methods. A series of straight-chain molecules ( dihydrogen dichalcogenide, dimethyl dichalcogenide, and (+)-bis(2-methylbutyl) dichalcogenide) and one cyclic molecule ( 2,3-(R,R)dichalcogenadecalin) were adopted for comparative analysis. The calculated excitation and circular dichroism ( CD) spectra were in good agreement with experimental ones (Laur, P. H. A. In Proceedings of the Third International Symposium on Organic Selenium and Tellurium Compounds; Cagniant, D., Kirsch, G., Eds.; Universite de Metz: Metz, 1979; pp 219-299) within 0.3 eV. The fitting CD spectra also reasonably reproduced the experimental ones. In all the molecules adopted, the first and second lowest bands were assigned to the n-sigma*(Ch-Ch) transition and the third and fourth lowest bands to the n-sigma*(Ch-R) transition. The first and second lowest bands apparently depended on the R-Ch-Ch-R dihedral angle, suggesting that the orbital energies of two sigma*(Ch-Ch) change with the R-Ch-Ch-R dihedral angle. This calculated trend agrees with two empirical rules: the C-2 rule and the quadrant rule.