I’m simulating small organic molecules using the PCFF force field to compare shear vs. non-shear conditions at two pressures: 0.1 MPa and 5 GPa.
After equilibrating systems at each pressure, I ran:
Non-shear simulations using NVT, and
Shear simulations using fix deform (xy shear) with a Langevin thermostat.
I monitored per-atom forces (fx, fy, fz) from the trajectory.
Surprisingly, the range of atomic forces is little hard to believe here.
For both 0.1 MPa and 5 GPa systems:
The range of net atomic forces remains very similar under non-shear conditions.
The same is true under shear conditions.
The only clear distinction appears between non-shear vs. shear simulations — not between the two pressure conditions.
Is this expected behavior? Could it be related to how fix deform and the thermostat interact with the system?
Any insights or suggestions would be appreciated.
I was expecting that, under different pressure conditions, the atoms would exhibit varying magnitudes of force. Additionally, with increasing shear strain, there should be a noticeable change in the net forces acting on the atoms. However, in my simulations, the net atomic forces remain within a similar range across both pressure conditions and throughout the shear deformation.
At any rate, you have not given a justification for your expectation.
Consider that your atoms are at finite temperature and thus oscillating around their equilibrium positions. You seem to by applying reasoning that is only true at the macroscopic level to atom scale simulations. That won’t work. You will have to apply an average over time with a significantly large window to see anything.
To confirm, you could just do the math for a single pair of atoms oscillating with the energy equivalent to the temperature that you are applying and then compute the magnitude of the force impact on individual atoms from the pressure you apply.