Global triplet potential energy surfaces for the N2(X1Σ) + O(3P) → NO(X2Π) + N(4S) reaction

Abstract

This work presents two global triplet potential energy surfaces (PESs) for the high-energy reaction N2(X(1)Σ) + O((3)P) → NO(X(2)Π) + N((4)S)-in particular, for the lowest energy (3)A' and (3)A″ PESs. In order to obtain the energies needed for fitting analytic surfaces, we carried out multireference configuration interaction (MRCI) calculations based on wave functions obtained from state-averaged complete active space self-consistent field calculations for 2280 geometries for the three lowest (3)A″ states and for 2298 geometries for the three lowest (3)A' states. The lowest-energy (3)A' and (3)A″ states at each of these geometries were then improved by applying the dynamically scaled external correlation (DSEC) method to all MRCI points, and the resulting DSEC energies were used for construction of the ground-state PES for each symmetry. The many-body component of the DSEC energies for the three-dimensional (3)A' and (3)A″ PESs was then least-squares fitted in terms of permutationally invariant polynomials in mixed exponential-Gaussian bond order variables. The global and local minima as well as the transition structures of both the (3)A' and the (3)A″ analytic PES were explored. In agreement with previous work, we find that the reverse reaction is barrierless on the (3)A″ surface along the minimum energy pathway. However, we have explored several new local minima and transition structures on the (3)A' PES. Furthermore, based on the newly found minima and transition structures, two independent reaction mechanisms have been illustrated for the reaction path on the (3)A' PES. The analytic surfaces may be used for dynamics calculations of electronically adiabatic reactive scattering and energy transfer.

Reactive collisions for NO(2Π) + N(4S) at temperatures relevant to the hypersonic flight regime

“…The cusps are due to crossing with higher excited electronic states which are well captured by the RKHS representations but were one of the main causes of errors in earlier PESs. 26 The The experimental relative energies of the different channels can be compared with the analytical PESs, see Table 2. The present PESs underestimate the experimentally determined reaction energy (NO+N→N 2 +O) by less than 4% while for full atomization this value reduces to 1.5%.…”

“…This is in contrast to the most recent PES which used an empirical extrapolation procedure for matching the energies of the reactant and products to the experimental ones which is probably not valid unconditionally for the entire surface. 26 Here it was decided to avoid such an empirical approach as the long-term goal is geared towards probing the performance of such computations at a given level of theory which can eventually be improved by morphing. 42 Furthermore, it is not expected that these differences lead to quantitatively different results.…”

“…[21][22][23][24][25] An earlier PES, which has been considered to be the most accurate one until recently, 1 was based on a many-body expansion derived from a grid of ab initio energies computed at the CASPT2 level using a cc-pVTZ basis set. 23 Very recently, new PESs for the two lowest triplet states were determined at the multi reference configuration interaction (MRCI+Q) level of theory using the maug-cc-pVTZ basis set 26 and showed remarkable differences with the earlier PES. For example, the global minimum of the 3 A ′′ state was found to be 0.23 eV deeper than in the previous work.…”

“…However, the reverse NO+N−→N 2 +O reaction was not considered because these surfaces do not have the correct long-range radial dependence which is essential. 25,26 In the present work the PESs for the 3 A ′ and 3 A ′′ states of N 2 O are determined at the MRCI+Q/cc-pVTZ level of theory and represented as a reproducing kernel Hilbert space (RKHS). Such a treatment reproduces the points from electronic structure calculations exactly and captures the correct R −6 decay for dipole-induced dipole interactions.…”

Reactive molecular dynamics: From small molecules to proteins

“…Based on CASPT2 calculations a many‐body expansion was fitted and used for variational transition state calculations and QCT simulations . A more recent effort used a least‐squares fit of permutationally invariant polynomials to dynamically scaled MRCI energies . These PESs have then been used in subsequent QCT simulations .…”