The Lennard-Jones (LJ) potential of interaction between two molecules as a function of distance ($r$), is given by $V_{LJ}(r) = 4\epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right]$, where $\epsilon$ and $\sigma$ are constantsThe expression of $r$ at which $V_{LJ}(r)$ reaches minimum is _ _ _.