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Graphical abstract Highlights • New spectroscopically-determined potential energy surfaces for both HCN and HNC. • Accurate ab initio dipole moment surface for the HCN system. • Improved computed transition intensities, particularly for the n3 fundamental band, are presented. Abstract Calculations of transition intensities for small molecules like H 2 O, CO, CO 2 based on s high-quality potential energy surface (PES) and dipole moment surface (DMS) can nowadays reach sub-percent accuracy. An extension of this treatment to a system with more complicated internal structure – HCN/HNC (hydrogen cyanide/hydrogen isocyanide) is presented. A highly accurate spectroscopically-determined PES is built based on a recent ab initio PES of the HCN/HNC isomerizing system. 588 levels of HCN with J = (0, 2, 5, 9, 10) are reproduced with a standard deviation from the experimental values of σ = 0.0373 cm−1 and 101 HNC levels with J = (0, 2) are reproduced with σ = 0.37 cm−1. The dependence of the HCN rovibrational transition intensities on the PES is tested for the wavenumbers below 7200 cm−1. Intensities are computed using wavefunctions generated from an ab initio and our optimized PES. These intensities differ from each other by more than 1% for about 11% of the transitions tested, showing the need to use an optimized PES to obtain wavefunctions for high-accuracy predictions of transition intensities. An ab initio DMS is computed for HCN geometries lying below 11,200 cm−1. Intensities for HCN transitions are calculated using a new fitted PES and newly calculated DMS. The resulting intensities compare much better with experiment than previous calculations. In particular, intensities of the H C stretching and bending fundamental transitions are predicted with the subpercent accuracy. [ABSTRACT FROM AUTHOR]

Intensities of 14 lines in the sixth overtone (7–0) band of carbon monoxide (12C16O) are measured in the visible range between 14 300 and 14 500 cm−1 using a frequency-stabilized cavity ring-down spectrometer. This is the first observation of such a high and weak overtone spectrum of the CO molecule. A theoretical model is constructed and tested based on the use of a high accuracy ab initio dipole moment curve and a semi-empirical potential energy curve. Accurate studies of high overtone transitions provide a challenge to both experiment and theory as the lines are very weak: below 2 × 10−29 cm molecule−1 at 296 K. Agreement between theory and experiment within the experimental uncertainty of a few percent is obtained. However, this agreement is only achieved after issues with the stability of the Davidson correction to the multi-reference configuration interaction calculations are addressed. [ABSTRACT FROM AUTHOR]

• New dipole moment surface provides excellent calculated intensities. • Calculated line lists agrees very well with many experimental and theoretical sources. • Transition intensities are sensitive to subtle changes in the potential. • Further work is required in the visible. New line lists are presented for the two most abundant water isotopologues; H 2 16O and H 2 18O. The H 2 16O line list extends to 25710 cm − 1 with intensity stabilities provided via ratios of calculated intensities obtained from two different semi-empirical potential energy surfaces. The line list for H 2 18O extends to 20000 cm − 1. The minimum intensity considered for all is 10 − 30 cm molecule − 1 at 296 K, assuming 100% abundance for each isotopologue. Fluctuation of calculated intensities caused by changes in the underlying potential energy are found to be significant, particularly for weak transitions. Direct comparisons are made against eighteen different sources of line intensities, both experimental and theoretical, many of which are used within the HITRAN2016 database. With some exceptions, there is excellent agreement between our line lists and the experimental intensities in HITRAN2016. In the infrared region, many H 2 16O bands which exhibit intensity differences of 5–10% between to the most recent 'POKAZATEL' line list (Polyansky et al., [Mon. Not. Roy. Astron. Soc. 480 , 2597 (2018)] and observation, are now generally predicted to within 1%. For H 2 18O, there are systematic differences in the strongest intensities calculated in this work versus those obtained from semi-empirical calculations. In the visible, computed cross sections show smaller residuals between our work and both HITRAN2016 and HITEMP2010 than POKAZATEL. While our line list accurately reproduces HITEMP2010 cross sections in the observed region, residuals produced from this comparison do however highlight the need to update line positions in the visible spectrum of HITEMP2010. These line lists will be used to update many transition intensities and line positions in the HITRAN2016 database. [ABSTRACT FROM AUTHOR]