A computational study of the cyclopropanation reactions of divalent samarium carbenoid \SmCH2\ with ethylene is presented. The reaction proceeds through two competing pathways: methylene transfer and carbometalation. The \SmCH2\ species was found to have a "samarium carbene complex" character with properties similar to previously investigated lithium carbenoids (LiCH2X where X = Cl, Br, I). The \SmCH2\ carbenoid was found to be noticeably different in structure with more electrophilic character and higher chemical reactivity than the closely related classical Simmons-Smith (\ZnCH2\) carbenoid. The effect of THF solvent was investigated by explicit coordination of the solvent THF molecules to the Sm (II) center in the carbenoid. The \SmCH2\/(THF)(n) (where n = 0, 1, 2) carbenoid methylene transfer pathway barriers to reaction become systematically lower as more THF solvent is added (from 12.9 to 14.5 kcal/mol for no THF molecules to 8.8 to 10.7 kcal/mol for two THF molecules). In contrast, the reaction barriers for cyclopropanation via the carbometalation pathway remain high (>15 kcal/mol). The computational results are briefly compared to other carbenoid reactions and related species.