Hi,

I've constructed a model of a random co-polymer with a single chain of 400 repeat units and packed the chain into a periodic box with Amorphous cell at low density, i.e. below the experimental density. The low density is to prevent ring spearing and catenation as the repeat units contain cyclic ring structures. I then performed a series of dynamics and geometry optimization steps with Forcite using the COMPASS forcefield in order to relax the structure, and then using NPT dynamics slowly compressed the model to above the experimental density of the polymer. I then annealed the structure using the annealing dynamics option, after which I attempted to relax the structure to close to its experimental density using NPT dynamics with a set pressure of 1 bar (0.0001 GPa). After 500 ps with a time step of 0.5 fs however, the density was about 1.84 g/cm3, where the experimental density is reported as 1.74 g/cm3. Thus the density is stil a bit high. I then tried to anneal the structure again and to repeat the NPT relaxation step again using 1000 ps simulation time and a time step of 1 fs. The resulting density as a function of simulation time is shown in the picture, where this time it increased again to about 1.94 g/cm3. I am using the Berendsen barostat and Andersen Thermostat for the NPT runs and have noticed that the pressure is not acurately reproduced at 0.0001GPa. Is this because this level of accuracy is not achieveable?

Does anyone have any ideas as to what I can try to get closer to the experimental density?

Thanks