AN AB-INITIO DETERMINATION OF THE MAGNETIC HYPERFINE-STRUCTURE OF C-2IN THE 4 LOWEST TRIPLET-STATES

The magnetic hyperfine coupling constants of Ct are calculated for the four lowest triplet states ((3) Pi(u), (3) Sigma(g)(-), (3) Sigma(u)( +), (3) Pi(g)) using various ab initio methods. The dependence of the hfcc's on the computational method is carefully studied to obtain an i nsight into the applicability of approximate methods in the calculatio n of magnetic hfcc's for larger C-n clusters. Only MRD-CI/B-K, QCISD(T ), CCSD(T) and UMP4 are able to describe hfcc's accurately for all ele ctronic states of C-2 while more approximate approaches (e.g. DFT, UMP 2, UMP3, MRD-CI) fail in at least one state, The calculated values are compared with experimental data, and differences between the various electronic stales are discussed by means of orbital relaxation effects and spin polarisation mechanisms.