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An ultrasharp photoluminescence line intimately related to antiferromagnetic order has been found in NiPS_{3}, a correlated van der Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure using angle-resolved photoemission and q-dependent electron energy loss spectroscopy as experimental tools we develop, in a first step, an adequate theoretical understanding using density functional theory (DFT). In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our Letter connects the understanding of the electronic structure and of optical processes in NiPS_{3} and related materials as a prerequisite for further progress of the field.

We report a systematic elastoresistivity study on LaFe_{1-x}Co_{x}AsO single crystals, which have well separated structural and magnetic transition lines. All crystals show a Curie-Weiss-like nematic susceptibility in the tetragonal phase. The extracted nematic temperature is monotonically suppressed upon cobalt doping, and changes sign around the optimal doping level, indicating a possible nematic quantum critical point beneath the superconducting dome. The amplitude of the nematic susceptibility shows a peculiar double-peak feature. This could be explained by a combined effect of different contributions to the nematic susceptibility, which are amplified at separated doping levels of LaFe_{1-x}Co_{x}AsO.