To study the effects of doping concentration on elastic constants, I calculate the elastic constants for one element doping in a 16 atom cell (2x2x1 extension from the opriginal conventional unit cell). What I found is that the elastic constants are dramatically different if I double the cell (2x2x2 extension from original cell) and use two doping atoms. So the concentration is the same. ANd I put these two doping atoms as far awar as possible, My question is whether the elastic constants calculations should be done a cubic cell? If I use a rectanglular cell, the caluclations are then not accurate anymore? Or any specical treatment is required , e.g., the default strain can not be used? Thanks in advance for your help!

Leon.