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The conversion of ethylene to ethylidyne on Rh(111) is examined using self-consistent periodic density function theory. The adsorptions of the reactants, intermediates, and products involved as well as the thermodynamics and kinetics of the conversion are characterized. Ethylene could form two adsorption configurations designated as di-σ and π adsorptions on Rh(111); ethyl, vinyl, vinylidene, ethylidyne, and ethylidene prefer the saturated sp3 configuration of both carbon atoms with the lost H atoms replaced by the metal atoms. The three-step conversion path on Rh(111), i.e., ethylene → vinyl → vinylidene → ethylidyne, is the most feasible, in which the vinylidene hydrogenation is the rate-limiting step. The pathway through ethylidene intermediate, ethylene → vinyl → ethylidene → ethylidyne, is impractical because it has a conversion rate at least 104 times lower than the most favorable path at the real reaction conditions. The mechanism via ethyl intermediate, ethylene → ethyl → ethylidene → ethylidyne, is impossible because of the high dehydrogenation barrier of ethyl to ethylidene as well as the low barriers for the conversions of ethyl to ethane and/or ethylene. Conversion involving direct isomerizations is also unlikely to be important due to the very high energy barriers involved.