pressure decreases with increasing strain

Hi All,

I am trying to calculate the stress strain curve of a polymer system. I do this by deforming the box using fix deform command with an engineering strain rate of 2.0 along the X-axis and constant volume conditions. Also, I remap the position of the atoms, not their velocities. I use a nose hoover thermostat(NVT) during the deformation process. I then use the compute pressure command to output the total pressure.

A problem that I am facing is that my pressure values are decreasing with increasing strain. When I tried compressing the system along the X-axis keeping volume constant, the pressure showed an increasing trend. As I understand, the pressure experienced by the system when expanded or compressed with constant volume is similar. Why is it that the total pressure is dropping with increasing strain rates during expansion?

Any input would be appreciated.

Thanks,
Janani

Hi All,

I am trying to calculate the stress strain curve of a polymer system. I do
this by deforming the box using fix deform command with an engineering
strain rate of 2.0 along the X-axis and constant volume conditions. Also, I
remap the position of the atoms, not their velocities. I use a nose hoover
thermostat(NVT) during the deformation process. I then use the compute
pressure command to output the total pressure.

A problem that I am facing is that my pressure values are decreasing with
increasing strain. When I tried compressing the system along the X-axis
keeping volume constant, the pressure showed an increasing trend. As I
understand, the pressure experienced by the system when expanded or
compressed with constant volume is similar. Why is it that the total
pressure is dropping with increasing strain rates during expansion?

​why not? i would expect exactly that. if that wouldn't be the case, you
could not do a NPT ensemble simulation, for example.

axel.​