Accurate DMBE potential energy surface for the N(2D)+H2(1Σ+g) reaction using an improved switching function formalism

A single-sheeted double many-body expansion (DMBE) potential energy surface is reported for the 1 (2)A '' state of NH2. To approximate its true multi-sheeted nature, a novel switching function that imposes the correct behavior at the H-2(X (1)Sigma(+)(g)) + N(D-2) and NH(X (3)Sigma(-)) + H(S-2) dissociation limits has been suggested. The new DMBE form is shown to fit with high accuracy an extensive set of new ab initio points (calculated at the multi-reference configuration interaction level using the full valence complete active space as reference and aug-cc-pVQZ and aug-cc-pV5Z basis sets) that have been semiempirically corrected at the valence regions by scaling the n-body dynamical correlation terms such as to account for the finite basis set size and truncated configuration interaction expansion. A detailed study of the N(D-2)center dot center dot center dot H-2(X (1)Sigma(+)(g)) van der Waals region has also been carried out. These calculations predict a nearly free rigid-rotor with two shallow van der Waals wells of C-2v and C-infinity v symmetries. Such a result contrasts with previous cc-pVTZ calculations which predict a single T-shaped van der Waals structure. Except in the vicinity of the crossing seam, which is replaced by an avoided intersection, the fit shows the correct physical behavior over the entire configurational space. The topographical features of the new DMBE potential energy surface are examined in detail and compared with those of other potential functions available in the literature. Amongst such features, we highlight the barrier for linearization (11, 802 cm(-1)) which is found to overestimate the most recent empirical spectroscopic estimate by only 28 cm-1. Additionally, the T-shaped N(2D)center dot center dot center dot H-2 van der Waals minimum is predicted to have a well depth of 90 cm(-1), being 11 cm(-1) deeper than the C-infinity v minimum. The title DMBE form is therefore recommendable for dynamics studies of both non-reactive and reactive N(D-2) + H-2 collisions.