\begin{table}
\caption {Fitting formulae for momentum transfer coefficients as %\protect\linebreak
 function of $v_{\rm rms}$
(in km~s$^{-1}$).\label{rates_vd}}
\begin{tabular}{lll}
\hline\hline
Species $s,s^\prime$ &  $\langle\sigma v\rangle_{ss^\prime}$ & $v_{\rm rms}$ \\
        &  (cm$^3$~s$^{-1}$) & (km~s$^{-1}$) \\
\hline
HCO$^+$, H$_2$  & $2.40\times 10^{-9} v_{\rm rms}^{0.6}$  & $0.2\la v_{\rm rms}\la 5$\\
H$_3^+$, H$_2$  & $2.00\times 10^{-9} v_{\rm rms}^{0.15}$ & $1\la v_{\rm rms}\la 10$\\
H$^+$, H$_2$    & $1.35\times 10^{-9} v_{\rm rms}^{0.22}$ & $1\la v_{\rm rms}\la 10$\\
$e$, H$_2$      & $3.16\times 10^{-11} v_{\rm rms}^{1.3}$ & $20\la v_{\rm rms}\la 200$\\
C$^+$, H    & $1.74\times 10^{-9} v_{\rm rms}^{0.2}$ & $2\la v_{\rm rms}\la 20$\\
H$^+$, H    & $2.13\times 10^{-9} v_{\rm rms}^{0.75}$ & $v_{\rm rms} \gtrsim 1$ (GKS) \\
$e$, H      & $2.50\times 10^{-10} v_{\rm rms}^{1.2}\exp(-v_{\rm rms}/460)$ & $20\la v_{\rm rms}\la 600$\\
H$^+$, He   & $1.48\times 10^{-9} v_{\rm rms}^{-0.02}$ & $0.1\la v_{\rm rms} \la 10$  \\
$e$, He     & $7.08\times 10^{-11} v_{\rm rms}$ & $20\la v_{\rm rms} \la 500$  \\
\hline
\end{tabular}
\end{table}