Table 2: Fitting formulae for momentum transfer coefficients as function of $v_{\rm rms}$ (in km s-1).
Species $s,s^\prime$ $\langle\sigma v\rangle_{ss^\prime}$ $v_{\rm rms}$
  (cm3 s-1) (km s-1)

HCO+, H2		 $2.40\times 10^{-9} v_{\rm rms}^{0.6}$ $0.2\lesssim v_{\rm rms}\lesssim 5$
H3+, H2 $2.00\times 10^{-9} v_{\rm rms}^{0.15}$ $1\lesssim v_{\rm rms}\lesssim 10$
H+, H2 $3.89\times 10^{-9} v_{\rm rms}^{-0.02}$ $1\lesssim v_{\rm rms}\lesssim 10$
e, H2 $3.16\times 10^{-11} v_{\rm rms}^{1.3}$ $20\lesssim v_{\rm rms}\lesssim 200$
C+,H $1.74\times 10^{-9} v_{\rm rms}^{0.2}$ $2\lesssim v_{\rm rms}\lesssim 20$
H+,H $2.13\times 10^{-9} v_{\rm rms}^{0.75}$ $v_{\rm rms} \gtrsim 1$ (GKS)
e, H $2.50\times 10^{-10} v_{\rm rms}^{1.2}\exp(-v_{\rm rms}/460)$ $20\lesssim v_{\rm rms}\lesssim 600$
H+, He $1.48\times 10^{-9} v_{\rm rms}^{-0.02}$ $0.1\lesssim v_{\rm rms} \lesssim 10$
e, He $7.08\times 10^{-11} v_{\rm rms}$ $20\lesssim v_{\rm rms} \lesssim 500$


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