AbstractThe effect of a cubic crystal field on rare‐earth ions is calculated and presented in a graphical form representing the individual magnetic moment and Van Vleck susceptibility of the lowest‐lying sub‐state, forJ= 3.5; 4; 4.5; 6; 7.5; 8. This is a practical guide for analyzing the moderate‐field part of the magnetization. The results are given for the three principal directions. In order to analyze susceptibility measurements, the calculated Curie constant (or the Van Vleck susceptibility) of the fundamental multiplet is also repr