1 \documentclass[12pt
]{article
}
6 \phi\left(r
\right) & = &
4 \epsilon \left[ \left(
\frac{\sigma}{r
}\right)^
{12} -
7 \left(
\frac{\sigma}{r
}\right)^
6 \right] \\
8 E
\left(r
\right) & = &
\phi\left(r
\right) -
\phi\left(R_c
\right) -
\left(r - R_c
\right)
\left.
\frac{d
\phi}{d r
} \right|_
{r=R_c
} \qquad r < R_c