I am getting the bulk modulus from fitting the equation of states and calculating the elastic constant. It seems that there is some relationship between bulk modulus B and elastic constant Cij, B=Cij/9,where i,j run from 1 to 3. But my result is far away from this, say B=50 from fitting EOS and B=300 from elastic constant. Is this resonable or B=Cij/9 cann't be applied to some material(structure)? Thanks a lot!