Two sets of pure deuterium plasmas are simulated using UEDGE, one including atoms and molecules and the other including atoms only as separate fluid species. Simulation results in one and two dimensions are reported to assess the role of molecules in tokamak plasmas. It is shown that thermal coupling of the molecules to the plasma can be relevant for the onset of detachment under highly collisional conditions in simple geometries, but play a modest role at low collisionality. Ion-molecule equipartition presents an additional ion energy loss channel, dissipating ion energy that would otherwise heat the electrons by equipartition. The resulting reduced electron temperature increases the ion-electron recombination by an order of magnitude as the plasma density is increased, yielding earlier and deeper detachment.