The ExoMol database ( www.exomol.com ) provides molecular data for spectroscopic studies of hot atmospheres. While the data are intended for studies of exoplanets and other astronomical bodies, the dataset is widely applicable. The basic form of the database is extensive line lists; these are supplemented with partition functions, state lifetimes, cooling functions, Landé g-factors, temperature-dependent cross sections, opacities, pressure broadening parameters, k -coefficients and dipoles. This paper presents the latest release of the database which has been expanded to consider 80 molecules and 190 isotopologues totaling over 700 billion transitions. While the spectroscopic data are concentrated at infrared and visible wavelengths, ultraviolet transitions are being increasingly considered in response to requests from observers. The core of the database comes from the ExoMol project which primarily uses theoretical methods, albeit usually fine-tuned to reproduce laboratory spectra, to generate very extensive line lists for studies of hot bodies. The data have recently been supplemented by line lists derived from direct laboratory observations, albeit usually with the use of ab initio transition intensities. A major push in the new release is towards accurate characterisation of transition frequencies for use in high resolution studies of exoplanets and other bodies.

A new silicon monoxide (28Si16O) line list covering infrared, visible and ultraviolet regions called SiOUVenIR is presented. This line list extends the infrared EBJT ExoMol line list by including vibronic transitions to the A 1Π and E 1Σ+ electronic states. Strong perturbations to the A 1Π band system are accurately modelled through the treatment of 6 dark electronic states: C 1Σ−, D 1Δ, a 3Σ+, b 3Π, e 3Σ− and d 3Δ. Along with the X 1Σ+ ground state, these 9 electronic states were used to build a comprehensive spectroscopic model of SiO using a combination of empirical and ab initio curves, including the potential energy (PE), spin-orbit (SO), electronic angular momentum (EAM) and (transition) dipole moment curves. The ab initio PE and coupling curves, computed at the multireference configuration interaction (MRCI) level of theory, were refined by fitting their analytical representations to 2617 experimentally derived SiO energy levels determined from 97 vibronic bands belonging to the X–X, E–X and A–X electronic systems through the MARVEL procedure. 112 observed forbidden transitions from the C–X, D–X, e–X, and d–X bands were assigned using our predictions, and these could be fed back into the MARVEL procedure. The SiOUVenIR line list was computed using published ab initio transition dipole moments for the E–X and A–X bands; the line list is suitable for temperatures up to 10 000 K and for wavelengths longer than 140 nm. SiOUVenIR is available from www.exomol.com and the CDS database.

We present a unified variational treatment of the electric quadrupole (E2) matrix elements, Einstein coefficients, and line strengths for general open-shell diatomic molecules in the general purpose diatomic code Duo. Transformation relations between the Cartesian representation (typically used in electronic structure calculations) to the tensorial representation (required for spectroscopic applications) of the electric quadrupole moment components are derived. The implementation has been validated against accurate theoretical calculations and experimental measurements of quadrupole intensities of 1 H 2 available in the literature. We also present accurate electronic structure calculations of the electric quadrupole moment functions for the X 1 Σ + electronic states of CO and HF at the CCSD(T) and MRCI levels of theory, respectively, as well for the a 1 ∆ g -b 1 Σ + g quadrupole transition moment of O 2 with MRCI level of theory. Accurate infrared E2 line lists for 12 C 16 O and 1 H 19 F are provided. A demonstration of spectroscopic applications is presented by simulating E2 spectra for 12 C 16 O, H 19 F and 16 O 2 (Noxon a 1 ∆ g -b 1 Σ + g band).

O 2 is important for spectroscopic applications in the IR, Visible and UV regions. In this work eight lowest electronic states were studied using the CASSCF and MRCI methods and the AV5Z basis sets with the D 2 h point group symmetry, namely. Potential energy curves (PECs) for 8 electronic states and spin-orbit coupling, electronic angular moment and transition quadrupole moment curves for the five states X 3 Σ − g , a 1 ∆ g , b 1 Σ + g , d 1 Π g and C 3 Π g , were computed and used to predict rovibronic spectra and lifetimes of O 2 . Our aim is to construct an accurate ro-vibronic molecular line list for O 2 . This will require an empirical refinement of the ab initio curves and will be considered in our future work.