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It is well known that hydrogen promotes catalyst activity in Cr/PNP‐catalyzed ethylene tetramerization, but the mechanism of this effect is unclear. A density functional theory (DFT) study was conducted to explore this effect, and conformation changes were carefully taken into consideration to build a clear reaction pathway. Three components in the ethylene tetramerization catalytic cycle were examined in detail: the production of 1‐hexene from metallacycloheptane, the production of 1‐octene from metallacyclononane, and the formation of an active centre on the Cr/PNP catalyst. This result indicates that the formation of an active centre on the catalyst becomes more favorable upon the imposition of hydrogen, where hydrogen functions as a second ligand. This easing effect could be the key factor leading to the outperformed Cr/PNP catalyst activity in ethylene tetramerization. [ABSTRACT FROM AUTHOR]