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A new global and highly accurate ab initio dipole moment surface (DMS) for water vapour is presented. This DMS is based on a set of 17 628 multi-reference configuration interaction data points that were calculated with the aug-cc-pCV6Z basis set with the Douglas-Kroll-Hess Hamiltonian; tests are performed at several other levels of ab initio theory. This new “CKAPTEN” DMS improves agreement with recent experimental measurements compared with previous models that poorly predicted some bands in the infrared while also maintaining or improving on the agreement for all remaining strong lines. For high overtones located in both the visible and the near ultraviolet regions, our predicted intensities all lie within 10% of recent atmospheric observations. A crossing of energy levels in the ν1 fundamental and 2ν2 states is seen to offset transition intensities in the ν1 fundamental band; residual inaccuracies within the potential energy surface used is the cause of this problem. [ABSTRACT FROM AUTHOR]

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]

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 H2 16 is presented. It is suggested that the associated set of line intensities is the most accurate available to date for this species. [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]

Line lists are presented for six deuterated isotopologues of water vapor namely HD 16 O, HD 17 O, HD 18 O, D 2 16 O, D 2 17 O and D 2 18 O. These line lists are prepared using empirically-determined energy levels, where available, to provide transition frequencies and high-quality ab initio dipole moment surfaces to provide transition intensities. The reliability of the predicted intensities is tested by computing multiple line lists and analyzing the stability of the results. The resulting intensities are expected to be accurate to a few percent for well-behaved, stable transitions. Complete T = 296 K line lists are provided for each species. [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]

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]

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]

• 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]

Monitoring ozone concentrations in the Earth’s atmosphere using spectroscopic methods is a major activity which undertaken both from the ground and from space. However there are long-running issues of consistency between measurements made at infrared (IR) and ultraviolet (UV) wavelengths. In addition, key O 3 IR bands at 10 µm, 5 µm and 3 µm also yield results which differ by a few percent when used for retrievals. These problems stem from the underlying laboratory measurements of the line intensities. Here we use quantum chemical techniques, first principles electronic structure and variational nuclear-motion calculations, to address this problem. A new high-accuracy ab initio dipole moment surface (DMS) is computed. Several spectroscopically-determined potential energy surfaces (PESs) are constructed by fitting to empirical energy levels in the region below 7000 cm − 1 starting from an ab initio PES. Nuclear motion calculations using these new surfaces allow the unambiguous determination of the intensities of 10 µm band transitions, and the computation of the intensities of 10 µm and 5 µm bands within their experimental error. A decrease in intensities within the 3 µm is predicted which appears consistent with atmospheric retrievals. The PES and DMS form a suitable starting point both for the computation of comprehensive ozone line lists and for future calculations of electronic transition intensities. [ABSTRACT FROM AUTHOR]

Room temperature NH 3 absorption spectra recorded at the Kitt Peak National Solar Observatory in 1980 are analyzed. The spectra cover two regions in the visible: 15,200 – 15,700 cm − 1 and 17,950 – 18,250 cm − 1 . These high overtone rotation-vibration spectra are analyzed using both combination differences and variational line lists. Two variational line lists were computed using the TROVE nuclear motion program: one is based on an ab initio potential energy surface (PES) while the other used a semi-empirical PES. Ab initio dipole moment surfaces are used in both cases. 95 energy levels with J = 1 − 7 are determined from analysis of the experimental spectrum in the 5 ν NH (red) region and 46 for 6 ν NH (green) region. These levels span four vibrational bands in each of the two regions, associated with stretching overtones. [ABSTRACT FROM AUTHOR]

An absorption spectrum of H 2 16 O at 1950 K is recorded in a premixed methane/air flat flame using a cavity-enhanced optical frequency comb-based Fourier transform spectrometer. 2417 absorption lines are identified in the 6250–6670 cm − 1 region with an accuracy of about 0.01 cm − 1 . Absolute line intensities are retrieved using temperature and concentration values obtained by tunable diode laser absorption spectroscopy. Line assignments are made using a combination of empirically known energy levels and predictions from the new POKAZATEL variational line list. 2030 of the observed lines are assigned to 2937 transitions, once blends are taken into account. 126 new energy levels of H 2 16 O are identified. The assigned transitions belong to 136 bands and span rotational states up to J = 27 . [ABSTRACT FROM AUTHOR]

Ab initio calculations of water intensities are becoming mature and are claimed to have 1% accuracy in many cases. Experimental intensities with 1% accuracy can be achieved with some care. An intercomparison of ab initio against experimental water intensities is presented for a variety of infrared bands for H 2 16 O and some for H 2 18 O and H 2 17 O. A new calculated H 2 16 O line list is presented for which uncertainties in the ab initio line intensities are evaluated. Much of the data show agreement within 2% between ab initio and experiment, however, for some bands, notably those involving excitation of some stretching modes, there are larger offsets of up to 8% attributed to ab initio calculation errors but still within the uncertainty of the ab initio calculation. In the ν 1 fundamental band differences of between + 5 % and − 13 % are found which show systematic dependence on wavenumber, Δ K a , and Δ J , again attributable to ab initio calculation errors. In the ν 2 band, intensity-dependent differences up to 2% originate from the analysis of the experimental data. At present experiments are important to validate ab initio calculations but ab initio predictions can be very useful in validating the experiment. As the two procedures display significantly different systematic errors, it is suggested that combining both gives the best results; this study will also facilitate further improvements of the theoretical methodology. [ABSTRACT FROM AUTHOR]

Total internal partition sums (TIPS) are reported for 166 isotopologues of 51 molecules important in planetary atmospheres. Molecules 1 to 50 are taken from the HITRAN2016 list, and, in some cases, additional isotopologues are considered for some of the molecules. Molecules 51–53 are C 3 H 4 , CH 3 , and CS 2 , respectively. TIPS are not reported for the O atom and CF 4 ; thus, while there are 53 species in the list, data are reported for 51 molecules. The TIPS are determined by various methods from 1 K to a T max that ensures the TIPS reported have converged. These data are provided with HITRAN2016 and a new version of the TIPS code is available in both FORTRAN and python languages. [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]