2500 vibronic eigenstates of the NO$_3$ radical

TL;DR

This study computes over 2500 vibronic states of the NO$_3$ radical using advanced tensor network methods, revealing detailed spectral features and limitations of the Born-Oppenheimer approximation in atmospheric chemistry modeling.

Contribution

It introduces a full-dimensional diabatic potential including five electronic states and applies tensor network methods to significantly expand the computed vibronic spectrum.

Findings

Computed over 2500 vibronic states, 50 times more than previous work

Identified large symmetry-induced level splittings in antisymmetric bending modes

Showed significant errors in spectra when using the Born-Oppenheimer approximation

Abstract

The nitrate radical NO$_3$ plays an important role in atmospheric chemistry, yet many aspects of its coupled and anharmonic vibronic structure remain elusive. Here, using an accurate, coupled full-dimensional diabatic potential that includes five electronic states, we revisit the vibronic spectrum associated with the electronic $\tilde X ^2A_2'$ state. Using recently developed tensor network state methods, we are able to compute more than 2500 vibronic states, thereby increasing the number of computed full-dimensional states by a factor of 50, compared to previous work. While we obtain good agreement with experiment for most of the assigned vibronic levels, for several others, we observe striking disagreement. Further, for the antisymmetric bending motion we find remarkably large symmetry-induced level splittings that are larger than the zero-order reference. We discuss non-negligible nonadiabatic effects and show that the Born-Oppenheimer approximation leads to significant errors in the spectrum.