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Abstract: Transition state theory rate constants for the 8-step homogeneous Hg–Cl reaction mechanism were computed based on high level quantum chemistry calculations for the temperature range of 298–2000K. ECP basis sets were used for Hg and accurate all-electron basis sets with polarization and diffuse functions were used for Cl/O/H species. The quantum computational method for each reaction was chosen by validating the calculated values of properties such as molecular structure, vibration frequency and reaction enthalpy. Activation energies for the Hg+Cl+M reaction calculated using the QCISD and QCISD(T) methods were inconsistent with those expected for radical recombination reactions. The three-body Hg/HgCl recombination reactions with Cl were observed to be the fastest mercury–chlorine reactions. The rate constants of Hg/HgCl reactions with HOCl were faster or comparable to that with Cl2 whereas the reactions involving Hg/HgCl and HCl were the slowest. The conversion of Hg+ to Hg2+ is faster than the conversion of Hg0 to Hg+ suggesting that HgCl is a reactive intermediate under these conditions. [Copyright &y& Elsevier]