[en] The total internal partition function of ammonia (¹⁴NH₃) and phosphine (³¹PH₃) are calculated as a function of temperature by explicit summation of 153 million (for PH₃) and 7.5 million (for NH₃) theoretical rotation-vibrational energy levels. High accuracy estimates are obtained for the specific heat capacity, C_p, the Gibbs enthalpy function, gef, the Helmholtz function, hcf, and the entropy, S, of gas phase molecules as a function of temperature. In order to reduce the computational costs associated with the high rotational excitations, only the A-symmetry energy levels are used above a certain threshold of the total angular momentum number J. With this approach levels are summed up to dissociation energy for values of J_max=45 and 100 for ammonia (E_max=41 051 cm⁻¹) and phosphine (E_max=28 839.7 cm⁻¹), respectively. Estimates of the partition function are converged for all temperatures considered for phosphine and below 3000 K for ammonia. All other thermodynamic properties are converged to at least 2000 K for ammonia and fully converged for phosphine. - Highlights: • Partition functions are computed for ammonia and phosphine. • Calculations are performed by direct summation of millions of energy levels. • Thermodynamic properties are computed for a wide range of temperatures

[en] The electronic structure of the X "2Σ"+, A "2Π, and B "2Σ"+ states of aluminum monoxide (AlO) are studied via ab initio multi-reference configuration interaction calculations. Core correlation corrections, several basis sets, and active space choices are considered. Angular momentum and spin-orbit coupling terms are obtained at different levels of theory. The resulting ab initio curves are used to solve the associated rovibronic problem for the total angular momentum J up to 112.5 and then also refined by fitting to the experimental wavenumbers available in the literature, reproducing them with the root-mean-square error of 0.07 cm"−"1. Theoretical rovibronic energy levels of AlO in its X "2Σ"+, A "2Π, and B "2Σ"+ electronic states are presented including those from the X − B blue-green system

[en] The calcium monohydroxide radical (CaOH) is an important astrophysical molecule relevant to cool stars and rocky exoplanets, among other astronomical environments. Here, we present a consistent set of highly accurate rovibronic (rotation-vibration-electronic) energy levels for the five lowest electronic states ($\stackrel{~<!--\; ~\; -->}{X}{}^2{\mathrm{\Sigma <!--\; ??\; -->}}^{+}$, $\stackrel{~<!--\; ~\; -->}{A}{}^2\mathrm{\Pi <!--\; ??\; -->}$, $\stackrel{~<!--\; ~\; -->}{B}{}^2{\mathrm{\Sigma <!--\; ??\; -->}}^{+}$, $\stackrel{~<!--\; ~\; -->}{C}{}^2\mathrm{\Delta <!--\; ??\; -->}$, and $\stackrel{~<!--\; ~\; -->}{D}{}^2{\mathrm{\Sigma <!--\; ??\; -->}}^{+}$) of CaOH. A comprehensive analysis of the published spectroscopic literature on this system has allowed 1955 energy levels to be determined from 3204 rovibronic experimental transitions, all with unique quantum number labeling and measurement uncertainties. The data set covers rotational excitation up to J = 62.5 for molecular states below 29,000 cm⁻¹. The analysis was performed using the Measured Active Rotational-Vibrational Energy Levels algorithm, which is a robust procedure based on the theory of spectroscopic networks. The data set provided will significantly aid future interstellar, circumstellar, and atmospheric detections of CaOH, as well as assist in the design of efficient laser cooling schemes in ultracold molecule research and precision tests of fundamental physics.

[en] Extensive line lists generated as part of the ExoMol project are used to compute lifetimes for individual rotational, rovibrational and rovibronic excited states, and temperature-dependent cooling functions by summing over all dipole-allowed transitions for the states concerned. Results are presented for SiO, CaH, AlO, ScH, H_2O and methane. The results for CH_4 are particularly unusual with four excited states with no dipole-allowed decay route and several others, where these decays lead to exceptionally long lifetimes. These lifetime data should be useful in models of masers and estimates of critical densities, and can provide a link with laboratory measurements. Cooling functions are important in stellar and planet formation. (paper)

[en] The spectra (rotational, rotation–vibrational or electronic) of diatomic molecules due to transitions involving only closed-shell (¹Σ ) electronic states follow very regular, simple patterns and their theoretical analysis is usually straightforward. On the other hand, open-shell electronic states lead to more complicated spectral patterns and, moreover, often appear as a manifold of closely lying electronic states, leading to perturbed spectra of even greater complexity. This is especially true when at least one of the atoms is a transition metal. Traditionally these complex cases have been analysed using approaches based on perturbation theory, with semi-empirical parameters determined by fitting to spectral data. Recently the needs of two rather diverse scientific areas have driven the demand for improved theoretical models of open-shell diatomic systems based on an ab initio approach; these areas are ultracold chemistry and the astrophysics of ‘cool’ stars, brown dwarfs and most recently extrasolar planets. However, the complex electronic structure of these molecules combined with the accuracy requirements of high-resolution spectroscopy render such an approach particularly challenging. This review describes recent progress in developing methods for directly solving the effective Schrödinger equation for open-shell diatomic molecules, with a focus on molecules containing a transition metal. It considers four aspects of the problem: (i) the electronic structure problem; (ii) non-perturbative treatments of the curve couplings; (iii) the solution of the nuclear motion Schrödinger equation; (iv) the generation of accurate electric dipole transition intensities. Examples of applications are used to illustrate these issues. (topical review)

[en] A theoretical study of the interstitial molecular hydrogen in the silicon single-crystal is reported. H_2 and Si have been approximated as a rigid object and a static matrix, respectively. A five-dimensional numerical-analytical representation of an ab initio potential energy surface of the system has been constructed. This representation has been used to calculate rotational, translational, and roto-translational energy levels of the interstitial hydrogen, where three levels of theory, 2D, 3D, and 5D were considered. The potential energy surface, the band structure of energy levels, and the roto-translational states obtained are presented together with the symmetry analysis of the roto-translational wavefunctions

[en] We present the first variational calculation of the isotropic hyperfine coupling constant of the carbon-13 atom in the CH_3 radical for temperatures T = 0, 96, and 300 K. It is based on a newly calculated high level ab initio potential energy surface and hyperfine coupling constant surface of CH_3 in the ground electronic state. The ro-vibrational energy levels, expectation values for the coupling constant, and its temperature dependence were calculated variationally by using the methods implemented in the computer program TROVE. Vibrational energies and vibrational and temperature effects for coupling constant are found to be in very good agreement with the available experimental data. We found, in agreement with previous studies, that the vibrational effects constitute about 44% of the constant’s equilibrium value, originating mainly from the large amplitude out-of-plane bending motion and that the temperature effects play a minor role

[en] High-resolution absorption spectra of NH_3 in the region 500–2100 cm"−"1 at temperatures up to 1027 °C and approximately atmospheric pressure (1013±20 mbar) are measured. NH_3 concentrations of 1000 ppm, 0.5% and 1% in volume fraction were used in the measurements. Spectra are recorded in high temperature gas flow cells using a Fourier Transform Infrared (FTIR) spectrometer at a nominal resolution of 0.09 cm"−"1. Measurements at 22.7 °C are compared to high-resolution cross sections available from the Pacific Northwest National Laboratory (PNNL). The higher temperature spectra are analysed by comparison to a variational line list, BYTe, and experimental energy levels determined using the MARVEL procedure. Approximately 2000 lines have been assigned, of which 851 are newly assigned to mainly hot bands involving vibrational states as high as v_2=5. - Highlights: • Absorption spectrum of ammonia recorded at 1300 K. • 2000 line assignments, 851 newly assigned. • Several bands observed for the first time.

[en] High-resolution absorption spectra of NH_3 in the region 2100–5500 cm"−"1 at 1027 °C and approximately atmospheric pressure (1045±3 mbar) are measured. An NH_3 concentration of 10% in volume fraction is used in the measurements. Spectra are recorded in a high-temperature gas-flow cell using a Fourier Transform Infrared (FTIR) spectrometer at a nominal resolution of 0.09 cm"−"1. The spectra are analysed by comparison to a variational line list, BYTe, and experimental energy levels determined using the MARVEL procedure. 2308 lines have been assigned to 45 different bands, of which 1755 and 15 have been assigned or observed for the first time in this work. - Highlights: • Absorption spectrum of ammonia recorded at 1300 K. • 2038 line assignments, 1755 newly assigned. • 15 bands observed for the first time.

[en] The carbon dimer, the ¹²C₂ molecule, is ubiquitous in astronomical environments. Experimental-quality rovibronic energy levels are reported for ¹²C₂, based on rovibronic transitions measured for and among its singlet, triplet, and quintet electronic states, reported in 42 publications. The determination utilizes the Measured Active Rotational-Vibrational Energy Levels (MARVEL) technique. The 23,343 transitions measured experimentally and validated within this study determine 5699 rovibronic energy levels, 1325, 4309, and 65 levels for the singlet, triplet, and quintet states investigated, respectively. The MARVEL analysis provides rovibronic energies for six singlet, six triplet, and two quintet electronic states. For example, the lowest measurable energy level of the $\mathrm{a}{}^3{\mathrm{\Pi <!--\; ??\; -->}}_{\mathrm{u}}$ state, corresponding to the J = 2 total angular momentum quantum number and the F ₁ spin-multiplet component, is 603.817(5) cm⁻¹. This well-determined energy difference should facilitate observations of singlet–triplet intercombination lines, which are thought to occur in the interstellar medium and comets. The large number of highly accurate and clearly labeled transitions that can be derived by combining MARVEL energy levels with computed temperature-dependent intensities should help a number of astrophysical observations as well as corresponding laboratory measurements. The experimental rovibronic energy levels, augmented, where needed, with ab initio variational ones based on empirically adjusted and spin–orbit coupled potential energy curves obtained using the Duo code, are used to obtain a highly accurate partition function, and related thermodynamic data, for ¹²C₂ up to 4000 K.