Two-electron, fourth rank (k = 4), crystal field parameters are fitted
for four neodymium compounds : LiYF4: Nd3+, Nd2O2S, Y2O3: Nd3+ and Nd
2O3, displaying a strong experimental/calculated discrepancy for the H
-2(11/2) level. The final mean deviation for all the levels is 2 cm- 1
(20 %) lower than that attained with the standard (one-particle) theo
ry. The compound parameters b(q){k1k2)k involve odd and even combinati
ons of k1 and k2. The geometrical dependence (q) seems to indicate a d
rift towards a C(infinity v) site symmetry caused by axial ligands. Th
e diagonal matrix elements of the correlated crystal field are smaller
than the off-diagonal matrix elements except for twin terms.