1 \documentclass[12pt
]{article
}
6 q_iq_j
\left[ \frac{\mbox{erfc
} (
\alpha r)
}{r
} -
\frac{\mbox{erfc
} (
\alpha r_c)
}{r_c
} +
7 \left(
\frac{\mbox{erfc
} (
\alpha r_c)
}{r_c^
2} +
\frac{2\alpha}{\sqrt{\pi}}\frac{\exp (-
\alpha^
2 r^
2_c)
}{r_c
} \right)(r-r_c)
\right] \qquad r < r_c
