The DpdPair interaction implements the soft pair interaction that was first introduced by Groot and Warren for use in dissipative particle dynamics simulaiion. The potential energy $V(r)$ for a pair of nonbonded particles separated by a distance $r$ is given by

$V(r) = \frac{\epsilon}{2} \left ( 1 - \frac{ r}{\sigma} \right )^{2}$

for all $r < \sigma$ , and $V(r) = 0$ for all $r > \sigma$ . The cutoff for this potential is thus equal to $\sigma$ . As for all pair potentials, different values are defined for the parameters $\epsilon$ and $\sigma$ for each distinct pair of monomer types $i$ and $j$ .

Values of $\epsilon$ and $\sigma$ are denoted in the parameter file and source code by matrix-valued variables named "epsilon" and "sigma", respectively. The parameter file format for DpdPair is

epsilon Matrix<float>

sigma Matrix<float>

where epsilon and sigma are symmetric matrix in lower diagonal form. For example, for a system with two types of monomer, using the same range parameter $\sigma$ for all interactions, we might have:

epsilon   25.0     30.0
          30.0     25.0
sigma     1.0       1.0
          1. 0      1.0