# How can the torsional barrier of a polymer determined by molecular dynamics?

First consider a simple polymer, like polyacetylene:

Its torsional barrier can be determined by DFT calculations by considering "enough" units and performing a simple dihedral scan. "Enough" appears to be 5-7 units (J. Chem. Phys., 2014, 140, 054310)

But now I want to study a more complex polymer:

The same method does not work, because the structure is too flexible. Different values are obtained after multiple turns around the central unit. Is it possible to use molecular dynamics to find the effective/average rotation barrier?

My guess would be that we could simulate this polymer over a certain period of time, then make an histogram of the central dihedral angle. We could then calculate the torsional barrier by looking at the relative frequencies of the angles (e.g. if we get a flat distribution, the barrier is 0, if some angle is very disfavored, the barrier should be high. I'm not sure how to quantitatively calculate the barrier though)

• Hm, what does your MD say to polyacetylene? If it can reproduce the results from DFT, I say you have a force-field that might well be up to the task. – Karl May 2 '20 at 20:11
• Could you do multiple turns using DFT and then use a statistical analysis for each dihedral angle (mod 2$\pi$) to get an idea about the potential? – Paul May 4 '20 at 18:23
• @Karl Assuming the forcefield is not a problem, would the general concept be valid? I think that the ensemble used in the simulation would be critical, and I don't have too much experience with MD. – Raphaël May 4 '20 at 21:33
• @Paul That's a good idea, although I don't know if sufficient sampling could be obtained with DFT for relatively large polymers. That's why MD came to my mind. – Raphaël May 4 '20 at 21:33