  Stable recursive relations are presented for the numerical computation of the
integrals
  $$\int d{\bf r}_1 d{\bf r}_2 r_1^{l-1} r_2^{m-1} r_{12}^{n-1}
  \exp{\{-\alpha r_1 -\beta r_2 -\gamma r_{12}\}}$$
  ($l$, $m$ and $n$ integer, $\alpha$, $\beta$ and $\gamma$ real) when the
indices $l$, $m$ or $n$ are negative. Useful formulas are given for particular
values of the parameters $\alpha$, $\beta$ and $\gamma$.
