  Twisting and bending deformations are crucial to the biological functions of
microfilaments such as DNA molecules. Although continuum-rod models have
emerged as efficient tools to describe the nonlinear dynamics of these
deformations, a major roadblock in the continuum-mechanics-based description of
microfilaments is the accurate modeling of the constitutive law, which follows
from its atomistic structure and bond-stiffnesses. Since first-principle
derivation of the constitutive law from atomistic structure is impractical and
so are direct experimental measurements due to the small length-scales, a
natural alternative is to estimate the constitutive law from discrete-structure
simulations such as molecular-dynamics (MD) simulations. In this paper, we
present a two-step inverse method for estimating the constitutive law using rod
theory and data generated from discrete-structure simulations. We illustrate
the method on a filament with an artificial and simplistic discrete-structure.
We simulate its deformation in response to a prescribed loading using a
multi-body dynamics (MBD) solver. Using data generated from the MBD solver, we
first estimate the curvature of the filament and subsequently use it in the
two-step method to estimate the effective constitutive-law relationship between
the restoring moment and curvature. Finally, we also illustrate how the
estimated constitutive law can be tested under independent loading conditions.
