A detailed theoretical survey of the potential energy surface (PES) for the reaction CH₃S with HCS in gas phase is carried out at the QCISD(t)/6-311＋＋G(d,p)//MPW1PW91/6-311G(d,p) level. The geometries, vibrational frequencies, and energies of all stationary points involved in the title reaction are calculated at the MPW1PW91/6-311G(d,p) level. More accurate energy information is provided by single-point calculations at QCISD(t)/6-311＋＋G(d,p) level. Relationships of reactants, intermediates, transition states and products are confirmed by the intrinsic reaction coordinate (IRC) calculations. 8 intermediates and 9 transition states are located and a variety of possible reaction pathways are probed. The association of CH₃S with HCS is found to be a barrierless process. Firstly it forms the energy-rich adducts a (five-member ring-type structure) or c (chain structure) through the weak S…S bond. Then, from adducts a and c, the main product P1 (2CH₂S) can be produced, as well as secondary important products P2 (CH₃SH＋CS), P3 (CH₄＋CS₂) and P4 [CH₂(SH)CSH] via the different channels, i.e. hydrogen shift, dissociation and isomerizazion. The reaction pathway leading to the major product 2CH₂S is as follows: R→a→TSa/b→b→P1. The rate constant of step a→TSa/b→b is k₁^CVT/SCT＝1.75×10¹⁰T^0.65exp(－907.6/T) s^－1 by means of small-curvature tunneling correction in the temperature range of 200～2000 K. By the analysis of the potential energy surface, we can draw the conclusion that all the channels are exothermic reactions and the reaction heat of generating P1 is －165.55 kJ•mol^－1.

Key words: CH₃S, HCS, density functional theory (DFT), reaction mechanism, rate constant