The SlitExternal interaction implements a repulsive 9-3 confining potential in which confining forces act only along the z/2 direction. In a boundary of length $L$ in the $z$ direction, the potential confines particles to the domain $0 < z < L$ by putting repulsive walls as $z = 0$ and $z = L$ . Let d be the shortest distance to these walls, given by the lesser of z and L - z. The potential energy is a function of d, and is given by

$V(d) = \frac{4\pi \epsilon \rho\sigma^{3}} {45} \left ( \frac{\sigma^{9}}{d^{9}} - 7.5 \frac{\sigma^{9}}{d^{9}} + \frac{25}{\sqrt{10}} \right)$

for $d_{i} < d < d_{c}$ , where $d_{i} = 0.506079 \sigma$ , and where $d_{c}$ is a user provided cutoff. For $d > d_{c}$ , the potential and force both vanish: $ V(d) = 0 $ . For $d < d_{i}$ , $V(d) = V(d_{i})$ , and the force vanishes.

The parameter file format for SlitExternal is

epsilon             float
sigma               float
cutoff              float

The parameter $\rho$ has a hard-coded default value of $\rho \equiv 0.7$ .