Why are the elastic constants very different when changing the box size?

Dear Lammps developers and users,

I am trying to calculate the elastic constants using the script in the example/elastic folder. The elastic constants are the same with different box size. However, when I use an EAM potential for an BCC alloy, the elastic constants vary a lot with changing the box size. For example, when I use a 2*3*4 (lattice units) box, the elastic constants are C11= 131, C12=104, C44=60 and these value are reasonable. When I adopt a 12*13*14 box, the elastic constants are C11= 739, C12=422, C44= **-**36. Could you please tell me why this happen?

Best regrads,


Impossible to say with so little information. There could be a number of issues in how you set up the different systems.