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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]

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born-Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational-vibrational energy levels up to 25 000 cm-1 which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm-1. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm-1 for four HCN isotopologues (HCN, DCN, H13CN and HC15N) being reproduced with σ = 0.37 cm-1. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm-1, respectively. [ABSTRACT FROM AUTHOR]

An ab initio study of the rotation-vibrational spectrum of the electronic ground state of the (gas-phase) fluoronium ion H 2 F + is presented. A new potential energy surface (PES) and a new dipole moment surface (DMS) were produced and used to compute rotation-vibrational energy levels, line positions and line intensities. Our computations achieve an accuracy of 0.15 cm −1 for the fundamental vibrational frequencies, which is about 50 times more accurate than previous ab initio results. The computed room-temperature line list should facilitate the experimental observations of new H 2 F + lines, in particular of yet unobserved overtone transitions. The H 2 F + molecular ion, which is isoelectronic to water, has a non-linear equilibrium geometry but a low-energy barrier to linearity at about 6000 cm −1 . As a result the effects of so-called quantum monodromy become apparent already at low bending excitations. An analysis of excited bends in terms of quantum monodromy is presented. [ABSTRACT FROM AUTHOR]

An ab initio potential energy surface (PES) for gas-phase ammonia NH 3 has been computed using the methodology pioneered for water (Polyansky et al., 2013). Multireference configuration interaction calculations are performed at about 50 000 points using the aug-cc-pCVQZ and aug-cc-pCV5Z basis sets and basis set extrapolation. Relativistic and adiabatic surfaces are also computed. The points are fitted to a suitable analytical form, producing the most accurate ab initio PES for this molecule available. The rotation-vibration energy levels are computed using nuclear motion program TROVE in both linearised and curvilinear coordinates. Better convergence is obtained using curvilinear coordinates. Our results are used to assign the visible spectrum of 14 NH 3 recorded by Coy and Lehmann (1986). Rotation-vibration energy levels for the isotopologues NH 2 D, NHD 2 , ND 3 and 15 NH 3 are also given. An ab initio value for the dissociation energy D 0 of 14 NH 3 is also presented. [ABSTRACT FROM AUTHOR]

In order to promote the development of the passive DOAS technique the Multi Axis DOAS - Comparison campaign for Aerosols and Trace gases (MAD-CAT) was held at the Max Planck Institute for Chemistry in Mainz, Germany, from June to October 2013. Here, we systematically compare the differential slant column densities (dSCDs) of nitrous acid (HONO) derived from measurements of seven different instruments.We also compare the tropospheric difference of SCDs (delta SCD) of HONO, namely the difference of the SCDs for the non-zenith observations and the zenith observation of the same elevation sequence. Different research groups analysed the spectra from their own instruments using their individual fit software. All the fit errors of HONO dSCDs from the instruments with cooled large-size detectors are mostly in the range of 0.1 to 0.3 x 1015 molecules cm-2 for an integration time of 1 min. The fit error for the mini MAX-DOAS is around 0.7 x 1015 molecules cm-2. Although the HONO delta SCDs are normally smaller than 6 x 1015 molecules cm-2, consistent time series of HONO delta SCDs are retrieved from the measurements of different instruments. Both fits with a sequential Fraunhofer reference spectrum (FRS) and a daily noon FRS lead to similar consistency. Apart from the mini-MAX-DOAS, the systematic absolute differences of HONO delta SCDs between the instruments are smaller than 0.63 x 1015 molecules cm-2. The correlation coefficients are higher than 0.7 and the slopes of linear regressions deviate from unity by less than 16% for the elevation angle of 1°. The correlations decrease with an increase in elevation angle. All the participants also analysed synthetic spectra using the same baseline DOAS settings to evaluate the systematic errors of HONO results from their respective fit programs. In general the errors are smaller than 0.3 x 1015 molecules cm-2, which is about half of the systematic difference between the real measurements. The differences of HONO delta SCDs retrieved in the selected three spectral ranges 335-361, 335-373 and 335-390 nm are considerable (up to 0.57 x 1015 molecules cm-2) for both real measurements and synthetic spectra. We performed sensitivity studies to quantify the dominant systematic error sources and to find a recommended DOAS setting in the three spectral ranges. The results show that water vapour absorption, temperature and wavelength dependence of O4 absorption, temperature dependence of Ring spectrum, and polynomial and intensity offset correction all together dominate the systematic errors. We recommend a fit range of 335-373 nm for HONO retrievals. In such fit range the overall systematic uncertainty is about 0.87 x 1015 molecules cm-2, much smaller than those in the other two ranges. The typical random uncertainty is estimated to be about 0.16 x 1015 molecules cm-2, which is only 25% of the total systematic uncertainty for most of the instruments in the MAD-CAT campaign. In summary for most of the MAX-DOAS instruments for elevation angle below 5°, half daytime measurements (usually in the morning) of HONO delta SCD can be over the detection limit of 0.2 x 1015 molecules cm-2 with an uncertainty of ~0.9 x 1015 molecules cm-2. [ABSTRACT FROM AUTHOR]

H3+ is a ubiquitous and important astronomical species whose spectrum has been observed in the interstellar medium, planets and tentatively in the remnants of supernova SN1897a. Its role as a cooler is important for gas giant planets and exoplanets, and possibly the early Universe. All this makes the spectral properties, cooling function and partition function of H3+ key parameters for astronomical models and analysis. A new high-accuracy, very extensive line list for H3+ called MiZATeP was computed as part of the ExoMol project alongside a temperature-dependent cooling function and partition function as well as lifetimes for excited states. These data are made available in electronic form as supplementary data to this article and at www.exomol.com. [ABSTRACT FROM AUTHOR]

In order to promote the development of the passive DOAS technique the Multi Axis DOAS - Comparison campaign for Aerosols and Trace gases (MAD-CAT) was held at the Max Planck Institute for Chemistry in Mainz, Germany from June to October 2013. Here, we systematically compare the differential slant column densities (dSCDs) of nitrous acid (HONO) derived from measurements of seven different instruments. We also compare the tropospheric difference of SCDs (delta SCD) of HONO, namely the difference of the SCDs for the non-zenith observations and the zenith observation of the same elevation sequence. Different research groups analysed the spectra from their own instruments using their individual fit software. All the fit errors of HONO dSCDs from the instruments with cooled large-size detectors are mostly in the range of 0.1 to 0.3 x 1015 molecules cm-2 for an integration time of 1 min. The fit error for the mini MAX-DOAS is around 0.7 x 1015 molecules cm-2. Although the HONO delta SCDs are normally smaller than 6 x 1015 molecules cm-2, consistent time series of HONO delta SCDs are retrieved from the measurements of different instruments. Both fits with a sequential Fraunhofer reference spectrum (FRS) and a daily noon FRS lead to similar consistency. Apart from the mini-MAX-DOAS, the systematic absolute differences of HONO delta SCDs between the instruments are smaller than 0.63 x 1015 molecules cm-2. The correlation coefficients are higher than 0.7 and the slopes of linear regressions deviate from unity by less than 16% for the elevation angle of 1°. The correlations decrease with an increase of elevation angle. All the participants also analysed synthetic spectra using the same baseline DOAS settings to evaluate the systematic errors of HONO results from their respective fit programs. In general the errors are smaller than 0.3 x 1015 molecules cm-2, which is about half of the systematic difference between the real measurements. The differences of HONO delta SCDs between retrieved in the selected three spectral ranges 335-361 nm, 335-373 nm and 335-390 nm are considerable (up to 0.57 x 1015 molecules cm-2) for both real measurements and synthetic spectra. We performed sensitivity studies to quantify the dominant systematic error sources and to find a recommended DOAS setting in the three spectral ranges. The results show that water vapour absorption, temperature and wavelength dependence of O4 absorption, temperature dependence of Ring spectrum, and polynomial and intensity offset correction all together dominate the systematic errors. We recommend a fit range of 335-373 nm for HONO retrievals. In such fit range the total systematic uncertainty from different sources is about 0.87 x 1015 molecules cm-2, much smaller than those in the other two ranges. Meanwhile the systematic bias of the fitted from the simulated real HONO delta SCDs is also smallest in 335-373 nm (about 0.02 x 1015 molecules cm-2). The typical random uncertainty is estimated to be about 0.16 x 1015 molecules cm-2, which is only 25 % of the total systematic uncertainty for most of the instruments in the MAD-CAT campaign. In summary most of the MAX-DOAS instruments can well observe the signals of atmospheric HONO absorption in case of HONO delta SCDs higher than 0.2 x 1015 molecules cm-2. However, systematic uncertainties limit the reliability of the results. [ABSTRACT FROM AUTHOR]