Happy New Year. 

According to the previous discussion, I found that there are two scripts for calculating mechanical properties, one is to apply stress and the other is to apply strain. In order to avoid the problem of Poisson's ratio, I did not use the method of applying strain, and the result obtained by applying strain is not very ideal. However, when I use the script of applying stress (StressStrain), the bond length hardly changed during the whole stretching process, only the molecules are pulled apart. I know that the force field I use does not include chemical bond breaking, but I think maybe the bond length should change. For this reason, I output the energy change in the stretching process, and found that the bond length and bond angle seem to change little in the whole stretching process, and the only big change is the van der Waals force.

I used this method to calculate the mechanical properties of the coarse-grained model (with a little modification to the script). I thought that my force field might not be accurate, so I used an all atom model（polyoxyethylene_cinnamide.xsd）with compass ii ff, The energy still hasn't changed much. When I set the stress to 10GPa, the actual stress is only about 0.9gpa. I don't know where to break through. Maybe I can get some inspiration？