A realistic multi-sheeted potential energy surface for NO2(2A′) from the double many-body expansion method and a novel multiple energy-switching scheme

We report a new multi-sheeted double many-body expansion potential energy surface that reproduces most known topological features of the title system. Near spectroscopic accuracy is conveyed to the ground-state sheet of (2)A(1) (1 (2)A(') in C-s) symmetry in the vicinity of the minimum by merging it with a spectroscopically determined Taylor-series-expansion-type form via a novel multiple energy-switching scheme. A high energy ridge for C-2v insertion of N(S-4) into O-2(X (3)Sigma(g)(-)) has also been imposed to mimic the result of accurate ab initio complete active space self-consistent field and second-order perturbation theory on CAS wave function calculations carried out for such geometries. This ridge decreases for C-s geometries yielding a minimum barrier height for the N+O-2 reaction of 0.273 eV at a bent N-O-O structure defined by R-NO=3.107a(0), R-OO=2.513a(0), and angleNOO=113.5 deg. Both the location and height of this barrier are in good agreement with existing ab initio calculations and the recommended values. Another salient feature is a shallow minimum on the (A) over tilde B-2(2) potential energy surface that is separated from the absolute (X) over tilde (2)A(1) minimum by a conical intersection. Such a feature is accurately predicted by the newly reported ab initio calculations and well mimicked by the global double many-body expansion/energy-switching potential energy surface here reported. This is therefore commended both for spectroscopic and reactive dynamics studies on the title system. A final comment in relation to the conical intersection and the energy-switching scheme goes to the expected accuracy of current approaches for spectroscopically determined effective single-valued forms. (C) 2003 American Institute of Physics.