Rare-earth ions in wide bandgap hosts experience a range of effects from the crystal field that lower the symmetry and split the large angular momentum states common in free atoms with partially-filled 4f shells. I will describe some recent progress in calculating these splittings from first-principles theory, with a focus on erbium ions in gallium nitride and yttrium oxide. These doped systems have potential applications for novel emitters and also for quantum transduction between optical and microwave modes. I will also describe predictions of novel methods of measuring these features using spin-polarized transport. Work on local crystal field effects was supported by NSF-DMREF through DMR-1921877 and work on spin-polarized transport sensing of spin Hamiltonians was supported by DOE BES DE-SC0016379.