Charge transfer in ultrafast isomerization of acetylene ions

First-principle calculations are employed to investigate the ultrafast isomerization of the acetylene cation and dication. We use the time-dependent density functional theory together with the Ehrenfest dynamics to track the coupled electron-nuclear dynamics. For both the acetylene cation and the dication, we observe nonadiabatic behaviors during the isomerization. We find that the charge transfer not only alters the electronic structure through nonadiabatic transitions, but also plays a key role in the subsequent hydrogen migration. We show that nonadiabatic transitions affect the structural modification of the excited potential energy surface, and also facilitate the ultrafast isomerization through the creation of a channel of increased negative charge that facilitates the proton movement. For the acetylene cation, we find a timescale for hydrogen isomerization of 66 +/- 4 fs, which is consistent with previous pump-probe experiments and on-the-fly calculations. For the dication, we find nonadiabatic transitions occur before the isomerization and identify a similar channel for the proton. Moreover, we find the formation of vinylidene-like structures is always accompanied by a characteristic charge separation on the carbon skeleton. These heuristics will be useful in identifying tautomers and motivating the ultrafast charge-transfer detection methods for future experiments.