AbstractThe (2s + 2a) path for the dimerization of ethylene is found to be high energy valley leading into the (2s+2s) path just after this latter's transition state. A minimal basis set (STO‐3G) ab initio method is employed and supplemented by either a 15 or 55 configuration interaction. The (2s+2a) barrier is found to be 109 (107) kcal/mole with 15 (55) configurations as opposed to 82.5 (82.3) kcal/mole for the (2s+2s) barrier. This is to be compared to the two‐step barrier of 42.4 kcal/mole previously found with STO‐3G and 15 configurations.It is found that four cis‐substituted ethylenes serving as the antarafacial partners reduce the (2s+2a) barrier as the substituents become stronger electron donors or acceptors. Conversely, the (2s+2s) barrier rises with the strength of the substituent.