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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]

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]