Streitz-Mintmire Al2O3 elastic constant

The data.streitz is a pristine, un-minimized alpha alumina structure. Did you properly minimize the structure? Here is what I got for alpha alumina, after additional minimization before the EC calculations.

LAMMPS: C11=421; C12=196; C13=120; C44=474; C14=-88; C44=67; C66=89 , from the Streitz-Mintmire potential implemented in LAMMPS
Paper: C11=537; C12=180; C13=106; C33=509; C14=-30; C44=130; C66=179, which is from the 1994 Streitz Mintmire paper
DFT: C11=450; C12=148; C13=111; C33=442; C14= 21; C44=130; C66=151 , from DFT

It appears the elastic constants are not as bad as those from your results. Note that elastic constants are very sensitive to residual stress, which is sensitively dependent upon minimization algorithm/approach, code implementation, and applied strain.

Hope this helps,