get 1/3 P in each direction when setting P in x only and zero in y and z

The plots look normal. Pressures fluctuate a lot in MD, especially
in solids. Why do you think anything is wrong? And I don't see
why you are concerned with the pressure for one dimension. In a solid
the other dimensions will be coupled to it.

Steve

Dear Steve,

I am looking at crystal/liquid alloy interface system. You are right since in a liquid system if we initially apply different pressure in x/y/z, it will end up with uniform pressure due to Pascal's law.

Since LAMMPS is capable of applying different normal pressures in different directions, I just tried applying pressure in one direction and got surprised by the uniform pressure coming out.

Still I don't see why the pressure should be coupled in solid. Say, in uni-axial compression or tension, the normal pressure in the other two directions should be zero.

Eugene

you are correct that a solid can sustain different pressures in different
dimensions, but only within limits. If the difference is too large,
or you try to change things too fast, there will be coupling.

Steve