absolute reference via the unified scale following IUPAC recommendations

Following the IUPAC recommendations for "The Unified Scale for Referencing in NMR" (see http://dx.doi.org/10.1351/pac200173111795 and https://www2.chem.wisc.edu/~cic/nmr/Guides/Other/Xi_chem_shift_scale.pdf), using internal reference such as TMS for other nuclei than 1H is no longer recommended.

With NMRium (https://docs.nmrium.org/#8-referencing)one can easily reference to any internal standard. Absolute reference following the IUPAC recommendations would be a valuable new feature for NMRium.