Nitrogen K-edge X-ray absorption Spectra of Ammonium and Ammonia in Water Solution: Assessing the Performance of Polarizable Embedding Coupled Cluster Methods

As X-ray absorption measurements of solutes in liquid environment become increasingly accessible, there is a strong need for reliable theoretical methods to assist in the interpretation of the experimental data. Coupled cluster (CC) methods, considered among the most accurate tools in electronic structure theory to obtain molecular properties and spectra, have been extensively developed over the last decade to simulate X-ray absorption in the gas phase. Their performance for solvated species, on the other hand, has remained so far largely unexplored. Here, we investigate the current state of the art of CC modeling of N K-edge X-ray absorption of aqueous ammonia and ammonium, based on various levels of QM/MM. Both the level of CC calculations and polarizable embedding are scrutinized. The results are compared to existing experimental data as well as simulations based on transition-potential density functional theory results.