Dear LAMMPS users,

I am subjecting a polymer melt system (Kremer Grest model) to oscillatory shear using fix deform wiggle along with NVT/SLLOD. I am attaching a small portion of the script here for reference.

dt 0.001

fix f1 all nvt/sllod temp 1.0 1.0 0.1

fix 2 all deform 1 xy wiggle 0.1 6 remap v

compute press all pressure f1_temp

variable strain equal xy/lx

variable xy equal xy

fix f3 all ave/time 1 1 1 c_press[4] v_strain v_xy file response.txt start 360000

I further wish to evaluate G’ and G" from the well known relationship: (stress_xy) = G’*sin(omega*t) + G"*cos(omega*t). To do so, I compute pressure first, then print out the c_press[4]. Further, I average over many cycles (around 5000) and then curve fit the final averaged cycle to obtain G’ and G". I am attaching the averaged response cycle for your reference.

My issue is, when I curve fit the above equation, the G’ and G" both are turning out to be **negative** values. I am not sure where the error is. Also, as seen in the above diagram, the strain is lagging the stress (shouldn’t it be the other way around, since strain is the applied/forcing variable?) I am attaching the averaged response for your reference too.

response.txt.averaged (139.6 KB)

Awaiting suggestions, thank you.