A hybrid variational-perturbation calculation of the ro-vibrational spectrum of nitric acid

TL;DR

This paper presents a hybrid variational-perturbation method to accurately calculate the ro-vibrational spectrum of nitric acid up to 7000 cm⁻¹, achieving close agreement with experimental data and generating a comprehensive line list.

Contribution

It introduces a hybrid variational-perturbation approach combined with ab initio PES and DMF to model nitric acid spectra with high accuracy, including hot bands often missing in existing databases.

Findings

Average frequency deviation of 0.2 cm⁻¹ for fundamentals.

Calculated intensities match experimental values within 0.3%.

Generated a line list with 2 billion lines at room temperature.

Abstract

Rotation-vibration spectra of the nitric acid molecule, HNO\3, are calculated for wavenumbers up to 7000~\cm. Calculations are performed using a Hamiltonian expressed in internal curvilinear vibrational coordinates solved using a hybrid variational-perturbation method. An initial potential energy surface (PES) and dipole moment function (DMF) are calculated {\it ab initio} at the CCSD(T)/aug-cc-pVQZ level of theory. Parameters of the PES and DMF are varied to minimize differences between the calculated and experimental transition frequencies and intensities. The average, absolute deviation between calculated and experimental values is 0.2~\cm\ for frequencies in the fundamental bands and 0.4~\cm\ for those in the first overtone and lowest combination bands. For the intensities, the calculated and experimental values differ by 0.3\% and 40\% for the fundamentals and overtones, respectively. The optimized PES and DMF are used to calculate the room-temperature ro-vibrational spectrum. These calculation reproduce both the form of the absorption bands, and fine details of the observed spectra, including the rotational structure of the vibrational bands and the numerous hot absorption band. Many of these hot bands are found to be missing from the compilation in HITRAN. A room temperature line list comprising $2 \times 10^{9}$ lines is computed.