The FeneBond interaction implements a simple harmonic bond potential with a nonzero rest length. The potential energy $V(r)$ for two covalently bonded particles separated by a distance $r$ is given by

$V(r) = \frac{1}{2}\kappa R_{0}^{2} \ln \left ( 1 - \frac{r^{2}}{R_{0}^{2}} \right )$

for values of $r$ less than a cutoff $r_{c}$ . The cutoff distance is given by the value of $r$ at which the force magnitude $dV(r))/dr$ equals a cutoff value $f_{c}$ that is specified in the parameter file. For $r > r_{c}$ , the force is constant, and the potential is given by

$V(r) = V(r_c) + f_{c}(r - r_{c})$

Different values are defined for the parameters $\kappa$ and $r_{0}$ for each bond type, but a single value of the force cutoff $f_{c}$ is used for all bond types.

Values of the parameters $\kappa$ , $R_{0}$ are denoted in the parameter file and source code by array-valued variables named "kappa", and "r0", respectively. The force cutoff $f_{c}$ is give by a single variable named "forceCutoff". The parameter file format is:

kappa       Array<float>
r0          Array<float>
forceCutoff <float>

The parameter block for system with two bond types (nBondType=2) might thus look like:

kappa         30.0
              35.0
r0             1.5
               1.5
forceCutoff   50.0