A new ab initio potential energy surface for the collisional excitation of N$_2$H$^+$ by H$_2$

International audienceWe compute a new potential energy surface for the study of inelastic collisions between N2H+ and H2 molecules. A preliminary study of the reactivity of N2H+ with H2 shows that neglecting reactive channels in collisional excitation studies is valid at low temperatures. The four dimensional N2H+– H2 potential is obtained from electronic structure calculations using the coupled cluster with single, double and perturbative triple excitation [CCSD(T)] level of theory. The atoms are described by the augmented correlation consistent triple zeta basis set. Both molecules were treated as rigid rotors. The potential energy surface exhibits a well depth of ≃ 2530 cm−1. Considering this very deep well, it appears that converged scattering calculations that take into account both the rotational structure of the two molecules should be very difficult to carry out. To overcome this difficulty, the ”adiabatic-hindered-rotor” treatment, which allows para-H2(j = 0) to be treated as if it was spherical, was used in order to reduce the scattering calculations to a 2D problem. The validity of this approach is checked and we find that cross sections and rate coefficients computed from the adiabatic reduced surface are in very good agreement with the full 4D calculations