question about iso keywork in fix npt command

Dear all,

The lammps that I am using is 2016Nov17. I met a problem when I use fix npt command. In the manual, it’s said that

Coupling means two things: the instantaneous stress will be computed as an average of the corresponding diagonal components, and the coupled box dimensions will be changed together in lockstep, meaning coupled dimensions will be dilated or contracted by the same percentage every timestep.

However, when I use fix 1 all npt temp 1120 1120 0.02 iso 0.0 0.0 0.2

I output lx, ly and lz every timestep and I found that the three directions are not coupled at all. While x and z dimensions are contracted, y dimension is dilated, and the percentages are totally different. Here are the output:

| | | | | | | |

  • | - | - | - | - | - | - | - |

Is there anything wrong with lammps? Or I don’t understand the manually correctly?

Best wishes,


I know of no issues with the iso keyword. It works as advertised. Perhaps

you are doing something else in your script. Post a simple as possible example

that illustrates the issue and why you think it is not working correctly.