High-accuracy water potential energy surface for the calculation of infrared spectra

Abstract

Transition intensities for small molecules such as water and CO₂ can now be computed with such high accuracy that they are being used to systematically replace measurements in standard databases. These calculations use high-accuracy ab initio dipole moment surfaces and wave functions from spectroscopically determined potential energy surfaces (PESs). Here, an extra high-accuracy PES of the water molecule (H₂¹⁶O) is produced starting from an ab initio PES which is then refined to empirical rovibrational energy levels. Variational nuclear motion calculations using this PES reproduce the fitted energy levels with a standard deviation of 0.011 cm^-1, approximately three times their stated uncertainty. The use of wave functions computed with this refined PES is found to improve the predicted transition intensities for selected (problematic) transitions. A new room temperature line list for H₂¹⁶O is presented. It is suggested that the associated set of line intensities is the most accurate available to date for this species.This article is part of the theme issue 'Modern theoretical chemistry'.

Keywords: potential energy surfaces; transition intensities; vibration–rotation.

Figure 1.

Differences, per cent of the observed value, for transition intensities predicted using wave functions from the PES15k and Bubukina et al. [21]. The observed data are taken from the recent high-accuracy measurements by Sironneau & Hodges [50]. (Online version in colour.)