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tokens: to 1.99 × 10 −18 cm 3 s −1 for temperatures ranging from 300–14 000 K, respectively, and can be approximated (within 7 per cent) as for temperatures between 300 and 1000 K and for temperatures higher than 1000 K. Because we do not take into account the fine structure of the phosphorus atom (∼227 cm −1 ), our predicted rate coefficients are uncertain at temperatures lower than about 327 K. Moreover, the B 2 Σ + state has a barrier against dissociation into P( 4 S) + O( 3 P). This has already been seen in the theoretical works of Liu et al. ( 2013 ) and Izzaouihda et al. ( 2014 ). Due to the potential barrier, the rate coefficients decrease rapidly at low temperatures. However, the height of the barrier is about 2416.49 cm −1 at 4.8 a 0 , which is about 508.66 cm −1 lower than the value reported by Liu et al. ( 2013 ). Also, the barrier in our PO B 2 Σ + potential appears to be significantly lower than the one shown in fig. 1 of Izzaouihda et al. ( 2014 ). The uncertainty in the barrier height results in a significant uncertainty in the rate coefficients at low temperatures. Other sources of error at low temperatures are quantum mechanical effects such as tunnelling and shape resonance (Bain Bardsley 1972 ; Smith 1989 ; Gustafsson 2013 ). At 300 K, quantum mechanical calculations (Antipov et al. 2009 ; Franz, Gustafsson Nyman 2011 ) show that the classical calculations (Dalgarno, Du You 1990 ; Singh et al. 1999 ; Singh Andreazza 2000 ) underestimate the rate coefficients by about 25 per cent and one order of magnitude for CN (Singh Andreazza 2000 ) and CO (Dalgarno et al. 1990 ; Singh et al. 1999 ) formation, respectively. At temperatures higher than 1000 K, the quantum mechanical (Antipov et al. 2009 ; Gustafsson 2013 ) and semiclassical (Dalgarno et al. 1990 ; Singh et al. 1999 ; Singh Andreazza 2000 ) rate coefficients are in good agreement. In these studies, CN and CO formation was assumed to occur mainly through the A 2 Π and A 1 Π states, respectively. The former potential curve has no barrier, while the latter shows a potential hump. In semiclassical studies, Dalgarno et al. ( 1990 ) and Singh et al. ( 1999 ) used the same set of potentials for the A 1 Π and X 1 Σ + states of CO and different transition dipole moment functions. The rate coefficients obtained by these authors are in agreemnet with each other. However, in quantum mechanical calculations, Franz et al. ( 2011 ) used a different set of potentials with a low barrier height for the A 1 Π state of CO. For CO, the resonance contribution to the rate coefficients arises from quantum mechanical tunnelling through the barrier (Franz et al. 2011 ). At low temperatures, both quantum mechanical and semiclassical theories depend strongly on barrier height. As previously mentioned, the uncertainty in barrier height results in a significant uncertainty in the rate coefficients at low temperatures. Considering all these effects, mainly the uncertainty in barrier height, it is likely that