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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]