The matrix elements are calculated recursively according to Buckmaster et al..
See: W. Hergert, M. Geilhufe, Group Theory in Solid State Physics and Photonics. Problem Solving with Mathematica
H.A. Buckmaster, R. Chatterjee and Y. H. Shin,The application of tensor operators in the analysis of EPR and ENDOR spectra, physica status solidi (a), WILEY-VCH Verlag, 1972, 13, 9-50