Persistence Length, End-to-End Distance, and Structure of Coarse-Grained Polymers

Coarse-grained (CG)
polymer simulations can access longer times
and larger lengths than all-atom (AA) molecular dynamics simulations;
however, not all CG models correctly reproduce polymer properties
on all length scales. Here we coarse-grain atomistic position data
from polyethylene (PE) and polytetrafluoroethylene (PTFE) melt simulations
by combining λ backbone carbon atoms in a single CG bead. Resulting
CG variables have correlations along the chain backbone that depend
on the coarse-graining scale λ and are generally not reproduced
by independent bond-length, bond-angle and torsion-angle distributions.
By constructing distributions of CG variables equivalent to those
from simulated CG potentials we are able to evaluate the bond-orientation
correlation for different CG models at reduced computational cost.
CG models and potentials that include only nonbonded, bond-length,
and bond-angle interactions computed by Boltzmann inversion correctly
reproduce the CG variable distributions but do not necessarily reproduce
the chain stiffness, overestimating the persistence length Lp and end-to-end distance ⟨R2⟩1/2 with increasing λ. While
CG models that include an independent torsion angle match the bond-orientation
correlation and ⟨R2⟩1/2 better, only approximate models that include correlations
between bond and torsion angles match the true bond-orientation correlation.