Table 1: Fitting formulae for momentum transfer coefficients as a function of the gas temperature T (in K) and $\theta\equiv\log (T/\rm{K})$ for $v_{\rm d}=0$ .
Species $s,s^\prime$ $\langle\sigma v\rangle_{ss^\prime}$

(10^-9 cm³ s^-1)

HCO⁺, H₂ $T^{1/2}(1.476-1.409\theta+0.555\theta^2-0.0775\theta^3)$

H₃⁺, H₂ $2.693-1.238\theta+0.664\theta^2-0.089\theta^3$

H⁺, H₂ $1.003+0.050\theta+0.136\theta^2-0.014\theta^3$

e, H₂ $T^{1/2}(0.535+0.203\theta-0.163\theta^2+0.050\theta^3)$

C⁺, H $1.983+0.425\theta-0.431\theta^2+0.114\theta^3$

H⁺, H 0.649 T^0.375 (GKS)

e, H $T^{1/2}(2.841+0.093\theta+0.245\theta^2-0.089\theta^3)$

H⁺, He $1.424+7.438\times 10^{-6}T-6.734\times 10^{-9}T^2$

e, He 0.428 T^1/2

**Table 1:** Fitting formulae for momentum transfer coefficients as a function of the gas temperature T (in K) and $\theta\equiv\log (T/\rm{K})$ for $v_{\rm d}=0$ .
Species $s,s^\prime$	$\langle\sigma v\rangle_{ss^\prime}$
	(10^-9 cm³ s^-1)
HCO⁺, H₂	$T^{1/2}(1.476-1.409\theta+0.555\theta^2-0.0775\theta^3)$
H₃⁺, H₂	$2.693-1.238\theta+0.664\theta^2-0.089\theta^3$
H⁺, H₂	$1.003+0.050\theta+0.136\theta^2-0.014\theta^3$
e, H₂	$T^{1/2}(0.535+0.203\theta-0.163\theta^2+0.050\theta^3)$
C⁺, H	$1.983+0.425\theta-0.431\theta^2+0.114\theta^3$
H⁺, H	0.649 T^0.375 (GKS)
e, H	$T^{1/2}(2.841+0.093\theta+0.245\theta^2-0.089\theta^3)$
H⁺, He	$1.424+7.438\times 10^{-6}T-6.734\times 10^{-9}T^2$
e, He	0.428 T^1/2

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