Four angular overlap parameters e

σ

、e

π

、e

δ

and e

φ

for Eu

3+

ions in LaOX (

X

=Cl

Br

I) crystals for which the approximative point group symmetry is C

4

have been calculated for the first time by making use of Angular Overlap Model(AOM). And the energy levels were fitted. The angulap overlar parameters obtained are listed in Table 2. From this we can see that the values of parameter e

σ

which describes the an-tibonding effect a decrease with the increasing atomic number

that is

the order of ligands is Cl

Br

I. The order is Cl

I

Br for parameter e

π

. Four angular overlap parameters are all larger for ligand Cl than for corresponding ligands Br and I. From this result we can draw a conclusion that the possibility of covalency between 4f electron and ligand is somewhat larger than the other two compounds. But there is no predo-minant order of covalency between ligands Br and I. Parameters e

δ

and e

φ

also have significant contribution to the energy-level splitting and shifting. Errors will occur if we neglect them. In our calculations

the reasons for large deviation are as follows.1. Deviation of free-ion energy le els. Because we adopt the average value of experimental energy levels as the value of free-ion energy levels. For Eu

3+

ions

the spectral structure is very complicated

especially for

7

F

5

and

7

F

6

. The assignment of energy levels is very difficult. Hence

the error of experimental energy levels can influence the results of calculation.2. A few parameters can also cause the deviation.3. As for Eu

3+

:LaOX(

X

=Cl

Br

I) series

oxygen is also a kind of ligand besides Cl

Br and I. In our calculations we assume that the charge of all ligands is equal.In Tables 3

4

and 5

the energy levels obtained from our calculations are given

compared with experimentals. The assignments of Irreducible Representation(I.R.)are ours. They have a little difference with those of experiments (see References[10

11]).