Ultracold D + H2 ( <i>v</i> = 4, <i>j</i> = 0) reaction dynamics: Long-range interactions, diagonal Born–Oppenheimer correction, and a diabatic geometric phase treatment

Ultracold hydrogen-exchange reactions have long provided a key platform for identifying subtle quantum mechanical effects in triatomic systems. The long-range interactions, diagonal Born-Oppenheimer correction (DBOC), and geometric phase (GP) have been predicted to influence such reactions, yet their individual effects have not been investigated. Here, we investigate the D + H2(v = 4, j = 0) → H + HD reaction using time-dependent wave packet calculations that incorporate a newly constructed long-range potential, DBOC, and a diabatic GP treatment. The inclusion of the accurate long-range interactions substantially alters the cold reaction dynamics by removing spurious short-range barriers and strongly modifying partial-wave resonances, including a pronounced shift and nearly 70% enhancement of the J = 2 resonance. The DBOC produces a modest overall reduction in the reaction rate, whereas the GP introduces the dominant suppression and governs the interference pattern in both total and state-resolved rates. These results demonstrate that the accurate inclusion of long-range interactions, DBOC, and GP is essential for reliably describing ultracold hydrogen-deuterium exchange reactions, clarifying the distinct roles each contribution plays in the reaction dynamics.