Predicting and characterizing new molecular species at a high-level theoretical approach: HSeCl, HClSe, and SeCl

► Prediction of new molecular species: HSeCl, HClSe, SeCl. ► Very reliable and accurate results to guide the experimental search and characterization of these species. ► The theoretical approach assesses the role of core-valence versus valence-only correlation on the calculated properties. ► Anharmonic corrections to the harmonic frequencies are determined at a very high-level of theory. ► The energetic profile should help interpret the reactions of H2Se with halogens in experiments to pump a HCl chemical laser.