Non-adiabatic couplings and dynamics in proton transfer reactions of Hn+ systems: Application to H2 + H2+ → H + H3+ collisions

Analytical derivatives and non-adiabatic coupling matrix elements are derived for H-n(+) systems (n = 3-5). The method uses a generalized Hellmann-Feynman theorem applied to a multi-state description based on diatomics-in-molecules (for H-3(+)) or triatomics-in-molecules (for H-4(+) and H-5(+)) formalisms, corrected with a permutationally invariant many-body term to get high accuracy. The analytical non-adiabatic coupling matrix elements are compared with ab initio calculations performed at multi-reference configuration interaction level. These magnitudes are used to calculate H-2(v' = 0, j' = 0) + H-2(+) (v, j = 0) collisions, to determine the effect of electronic transitions using a molecular dynamics method with electronic transitions. Cross sections for several initial vibrational states of H-2(+) are calculated and compared with the available experimental data, yielding an excellent agreement. The effect of vibrational excitation of H-2(+) reactant and its relation with non-adiabatic processes are discussed. Also, the behavior at low collisional energies, in the 1 meV-0.1 eV interval, of interest in astrophysical environments, is discussed in terms of the long range behaviour of the interaction potential which is properly described within the triatomics-in-molecules formalism. (C) 2015 AIP Publishing LLC.