An ab initio determination of the magnetic hyperfine structure of C2 in the four lowest triplet states

The magnetic hyperfine coupling constants of C2 are calculated for the four lowest triplet states (3Πu, 3Σg−, 3Σu+, 3Πg) using various ab initio methods. The dependence of the hfccʹs on the computational method is carefully studied to obtain an insight into the applicability of approximate methods in the calculation of magnetic hfccʹs for larger Cn clusters. Only MRD-CI/BK, QCISD(T), CCSD(T) and UMP4 are able to describe hfccʹs accurately for all electronic states of C2 while more approximate approaches (e.g. DFT, UMP2, UMP3, MRD-CI) fail in at least one state. The calculated values are compared with experimental data, and differences between the various electronic states are discussed by means of orbital relaxation effects and spin polarisation mechanisms.