Does the diffuse OH-stretch band in the IR spectrum of protonated oxalate exhibit quantum chaos?

We report and analyze vibrational self-consistent field and configuration interaction calculations of the infrared spectrum of protonated oxalate using precise machine-learned fits to electronic energies and dipole moment surfaces. For these fits, we use recently reported data by Meuwly and co-workers. Δ-machine learning is employed to elevate the MP2-level potential to the CCSD(T)/aug-cc-pVTZ level. In contrast to vibrational second-order perturbation theory and molecular dynamics simulations of this well-studied anion, the highly dispersed band in the expected OH-stretch region of the experimental spectrum is captured both in breadth and position. Calculations are also done for deuterated and tritiated isotopologs. The analysis of the eigenstates suggests that this band may exhibit "quantum chaos."