these $g$-factors and this has currently recently been implemented within [Duo]{} [@jtgfac]. The magnetic fields in some astronomical environments are strong enough that the Paschen-Back effect becomes important [@13KuBexx.CrH; @07ShFlBe.CN]. In this case the shifts in the energy levels do not have a simple dependence on the magnetic field [@05BeBrFl.diatom] and they will have to be explicitly computed on a case-by-case basis. Landé $g$-factors are only given for systems whose spectroscopic model includes open shell electronic states (ie ones with unpaired electrons) since these are the systems whose states which show significant splitting in a magnetic field.

Besides (approximate) quantum numbers, which may appear in several forms, it is also often desirable to store more than one estimate of the energy level of the state. A general methodology, known as MARVEL, is available for obtaining self-consistent sets of empirical energy levels from networks of transitions [@jt412; @12FuCsi.method], which means that sets of empirical energy levels are available for key molecules [@jt454; @jt482; @jt539; @jt576; @13FuSzMa.H3+; @jtC2]. In this case it is preferable to replace computed energy levels with empirical ones, as has already been done for the HCN/HNC [@jt570] and the CS [@jt615] line lists. It is also recommended to 