I'd like to deform a orthogonal simulation box in a way that the stress ratios between the three principle directions remain constant during the whole loading process. That is if the stress is Px, Py, Pz in x,y,z direction, I want to keep Py/Px and Pz/Px constant. It seems that "fix deform" command will not do the trick since it could only control the strain rate rather than stress increment. I'm wondering if there is such command in lammps for this kind of loading condition? Any suggestions will be greatly appreciated. Thank you.