Potential

The WcaPair interaction implements a Weeks-Chandler-Anderson (WCA) potential. This is a cutoff and shifted Lennard-Jones (LJ) interaction that is cutoff at the minimum of the LJ potential, which occurs at $r = \sigma 2^{1/6}$ . The WcaPair class is thus equivalent to a LJPair potential with a cutoff $r_{c}$ given by

$r_{c} \equiv 2^{1/6} \sigma$

This yields a potential energy for each pair of atom types

$V(r) = 4 \epsilon \left [ \left ( \frac{\sigma}{r} \right )^{12} - \left ( \frac{\sigma}{r} \right )^{6} + \frac{1}{4} \right ]$

for all $r < r_{c}$ , and $V(r) = 0$ for $r > r_{c}$ .

Parameters

The parameter file format is similar as that for an LJPair, except that no input value is required for the cutoff variable. Values of the parameters $\epsilon$ and $\sigma$ are denoted in the parameter file and source code by matrix-valued variables named "epsilon" and "sigma". The parameter file format is:

epsilon Matrix<float>

sigma Matrix<float>

For example, for a system with two types of monomer, we might have:

epsilon   1.0      1.5
          1.5      1.0
sigma     1.0      1.0
          1. 0     1.0

See also: LJPair