Using the unified ligand-field-coupling scheme, the 210210 complete energy matrices including all the spin states for d4 configuration transition metal ions are constructed within a strong field representation. By diagonalizing the complete energy matrices, the local lattice structure and the Jahn-Teller energy of Cr2+ ions doped into ZnS are investigated. It is found that the theoretical results are in good agreement with the experimental data. Moreover, the contribution of the spin singlet to the zero-field splitting (ZFS) parameter of Cr2+ ions doped into ZnS is also investigated. The results indicate that the spin singlet contribution to ZFS parameter D is negligible, but the contribution to ZFS parameters a and F may not be neglected.