The computations reveal that before inclusion of spin-orbit coupling, there are two low-lying quartet states separated by 477 cm -1 (Fig. ). The spin orbit coupling between these quartets is dominated by the z-component of the spin-orbit operator and amounts to an absolute value of 360 cm -1 . Therefore, we attribute the large ZFS of 1 to this strong first-order coupling of the two energetically close-lying quartet states. Indeed, in numerical experiments, in which this particular spin-orbit coupling matrix element was switched off, the ZFS reduced to around 30 cm -1 with positive D value for both quartet states.