“…In addition, some of the above-listed bands are observed in nominally "pure" synthetic samples (e.g., Turner and Crawford, 1975;Engstrom, 1980;Ramírez et al, 2004), and their relation to specific impurities or defects occurring at very low concentration levels may prove to be uncertain. To this end, a theoretical approach linking the microscopic structure of the defects to their spectroscopic properties could be useful, as attested by theoretical investigations of OH groups in nominally anhydrous silicates (e.g., Wright, 2006;Balan et al, 2017Balan et al, , 2020Blanchard et al, 2017;Jollands et al, 2020). Previous theoretical studies have focused on the association of hydrogen with intrinsic defects in corundum (Zhang et al, 2014;T-Thienprasert et al, 2017).…”