The absorption wavelengths of sulfur chromophors of ultramarines calculated by time-dependent density functional theory

The electronic structure of cis and trans tetrasulfur is discussed in terms of ylidic and biradical resonance structures by natural resonance theory. The singlet ground states display RKS/UKS instability. The broken-symmetry UKS structure is of lower energy than the RKS structure, which does, however, not influence the molecular and electronic structure significantly.