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A two-body gas-phase reaction rate coefficient can be given by the usualArrhenius-type formula which depends on temperature. The UMIST Database forAstrochemistry is a widely used database for reaction rate coefficients. Theyprovide fittings for coefficients valid over a particular range oftemperatures. The permissible upper-temperature limits vary over a wide range:from 100 K to 41000K. A wide range of temperatures occurs in nature; thus, itrequires evaluating the rate coefficients at temperatures outside the range ofvalidity. As a result, a simple extrapolation of the rate coefficients can leadto unphysically large values at high temperatures. These result in unrealisticpredictions. Here we present a solution to prevent the gas-phase reactioncoefficients from diverging at a very high temperature. We implement this intothe spectral synthesis code CLOUDY which operates over a wide range oftemperatures from CMB to 10$^{10}$ K subject to different astrophysicalenvironments.

Revisiting the gas-phase chemical rate coefficients at high temperatures in CLOUDY