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In this work, we report rovibrational energy levels for four isotopologues of methylene (CH2, CHD, CD2, and 13CH2) in their ground triplet electronic state (X~3B1) from variational calculation up to ~10,000 cm−1 and using a new accurate ab initio potential energy surface (PES). Triplet methylene exhibits a large‐amplitude bending vibration and can reach a quasilinear configuration due to its low barrier (~2000 cm−1). To construct the ab initio PES, the Dunning's augmented correlation‐consistent core‐valence orbital basis sets were employed up to the sextuple‐ζ quality [aug‐cc‐pCVXZ, X = T, Q, 5, and 6] combined with the single‐ and double‐excitation unrestricted coupled cluster approach with a perturbative treatment of triple excitations [RHF‐UCCSD(T)]. We have shown that the accuracy of the ab initio energies is further improved by including the corrections due to the scalar relativistic effects, DBOC and high‐order electronic correlations. For the first time, all the available experimental rovibrational transitions were reproduced with errors less than 0.12 cm−1, without any empirical corrections. Unlike more "traditional" nonlinear triatomic molecules, we have shown that even the energies of the ground vibrational state (000) with rather small rotational quantum numbers are strongly affected by the very pronounced rovibrational resonance interactions. Accordingly, the polyad structure of the vibrational levels of CH2 and CD2 was analyzed and discussed. The comparison between the energy levels obtained from the effective Watson A‐reduced Hamiltonian, from the generating‐function approach and from a variational calculation was given. [ABSTRACT FROM AUTHOR]

We introduce CaSDa24, the latest iteration of our high-resolution molecular spectroscopy database cluster. Comprising ten independent databases, each dedicated to a specific molecule (CH 4 , CH 3 Cl, SF 6 , C 2 H 4 , CF 4 , GeH 4 , RuO 4 , SiF 4 , UF 6 , SiH 4), CaSDa24 integrates updated and new databases since our previous publication. These databases include detailed lists of calculated spectral lines, eigenvectors, energy levels, and additional relevant information derived from the latest experimental spectra analyses. This article provides an overview of the current state of molecular spectroscopy databases, explains the foundational principles of CaSDa24, and summarizes the significant updates and new data introduced. • Calculated Spectroscopic Database: CaSDa24. • Major updates of existing databases. • New databases built on the same model. • Planetary and atmospheric applications. [ABSTRACT FROM AUTHOR]

In the present investigation, we report new analytical potential energy curves (APECs) for three triplet electronic states X 3 Σ - , B 3 Σ - , and C 3 Π of sulfur monoxide. For such, ab initio electronic energies have been calculated employing the highly accurate multireference configuration interaction method with Davidson modification associated with aug-cc-pV(5+d)Z basis set. The novel analytical representations have been constructed by fitting such accurate energies utilizing a least-squares fitting procedure. By considering our APECs, molecular properties as spectroscopic parameters, rovibrational levels, and classical turning points were derived. Our results show good agreement when compared with available experimental and theoretical data. The vibronic (vibration–electronic) transition parameters like Franck–Condon factors, Einstein coefficients, transition dipole moment, and r-centroids for the B 3 Σ - - X 3 Σ - band system were also examined. The present findings provide a reliable theoretical reference that could be employed in interpretations of astrochemical and astrophysical observations. Based on the results, we recommend these functional forms to represent the two-body fragments within the double many-body expansion method to develop potential energy surfaces for larger S-bearing species. [ABSTRACT FROM AUTHOR]

In this work, we report rovibrational energy levels for four isotopologues of methylene (CH 2 , CHD, CD 2 , and 13 CH 2 ) in their ground triplet electronic state ( X ~ 3 B 1 ) from variational calculation up to ~10,000 cm -1 and using a new accurate ab initio potential energy surface (PES). Triplet methylene exhibits a large-amplitude bending vibration and can reach a quasilinear configuration due to its low barrier (~2000 cm -1 ). To construct the ab initio PES, the Dunning's augmented correlation-consistent core-valence orbital basis sets were employed up to the sextuple-ζ quality [aug-cc-pCVXZ, X = T, Q, 5, and 6] combined with the single- and double-excitation unrestricted coupled cluster approach with a perturbative treatment of triple excitations [RHF-UCCSD(T)]. We have shown that the accuracy of the ab initio energies is further improved by including the corrections due to the scalar relativistic effects, DBOC and high-order electronic correlations. For the first time, all the available experimental rovibrational transitions were reproduced with errors less than 0.12 cm -1 , without any empirical corrections. Unlike more "traditional" nonlinear triatomic molecules, we have shown that even the energies of the ground vibrational state (000) with rather small rotational quantum numbers are strongly affected by the very pronounced rovibrational resonance interactions. Accordingly, the polyad structure of the vibrational levels of CH 2 and CD 2 was analyzed and discussed. The comparison between the energy levels obtained from the effective Watson A-reduced Hamiltonian, from the generating-function approach and from a variational calculation was given., (© 2023 Wiley Periodicals LLC.)

APUAMA is a free software designed to determine the reaction rate and thermodynamic properties of chemical species of a reagent system. With data from electronic structure calculations, the APUAMA determine the rate constant with tunneling correction, such as Wigner, Eckart and small curvature, and also, include the rovibrational level of diatomic molecules. The results are presented in the form of Arrhenius-Kooij form, for the reaction rate, and the thermodynamic properties are written down in the polynomial form. The word APUAMA means 'fast' in Tupi-Guarani Brazilian language, then the code calculates the reaction rate on a simple and intuitive graphic interface, the form fast and practical. As program output, there are several ASCII files with tabulated information for rate constant, rovibrational levels, energy barriers and enthalpy of reaction, Arrhenius-Kooij coefficient, and also, the option to the User save all graphics in BMP format. [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]

Results of high level ab initio calculations of the intermolecular potentials and theoretical dispersion coefficients for the Ne2 and Ar2 dimers were utilized to build analytical potentials for these species. The obtained potentials were used in the calculation of the dimers rovibrationals levels, and their respective spectroscopic constants determined. A comparison of high level experimental data with our theoretical results shows a very good agreement for Ne2, and also a good agreement for the Ar2 dimer.

Abstract: The far-infrared and middle-infrared emission spectra of deuterated water vapour were measured at temperatures 1370, 1520, and 1940K in the ranges 320–860 and 1750–3400cm−1. The measurements were performed in an alumina cell with an effective length of hot gas of about 50 cm. More than 3550 new measured lines for the D2 16O molecule corresponding to transitions from highly excited rotational levels of the (020), (100), and (001) vibrational states are reported. These new lines correspond to rotational states with higher values of the rotational quantum numbers compared to previously published determinations: J max =29 and K a (max) =22 for the (020) state, J max =29 and K a (max) =25 for the (100) state, and J max =30 and K a (max) =23 for the (001) state. The extended set of 1987 experimental rotational energy levels for the (020), (100), and (001) vibration states including all previously available data has been determined. For the data reduction we used the generating function model. The root mean square (RMS) deviation between observed and calculated values is 0.004cm−1 for 1952 rovibrational levels of all three vibration states. A comparison of the observed energy levels with the best available values from the literature and with the global predictions from molecular electronic potential energy surfaces of water isotopic species [H. Partridge, D.W. Schwenke, J. Chem. Phys. 106 (1997) 4618] is discussed. The latter confirms a good consistency of mass-dependent DBOC corrections in the PS potential function with new experimental rovibrational data. [Copyright &y& Elsevier]

• Spectroscopic properties for five low-lying electronic states of N 2 are obtained using ab initio based analitical potential energy curves. • Franck-Condon factors, r-centroids, Einstein coefficients and radiative lifetimes for the second positive ((C 3 Π u − B 3 Π g)) and Herman infrared (C ″ 5 Π u − A ′ 5 Σ g +) bands are reported. • Perturbations in the spectrum of the second positive system can be attributed to N 2 (C″5Π u and N 2 A ′ 5 Σ g + state. We present a detailed investigation about the vibronic (vibration-electronic) transition parameters as Franck-Condon (FC) factors, r-centroids, Einstein coefficients, and radiative lifetimes for some bands of the second positive (C 3 Π u − B 3 Π g) and Herman infrared (C ″ 5 Π u − A ′ 5 Σ g +) band systems of N 2. For such, novel and accurate functional forms are reported. To achieve our goals, ab initio electronic energies have been obtained employing the highly accurate multireference configuration interaction (MRCI) method with Davidson modification associated with aug-cc-pVXZ (X = 5,6) basis set. The present analytical representations were 'constructed by fitting such accurate energies utilizing a least-squares fitting procedure. By considering our Analytical Potential Energy Curves (APECs), we derived molecular properties as spectroscopic parameters, rovibrational levels, and classical turning points. Our results are compared with available experimental and theoretical data. In addition, we demonstrate that observed perturbations in the experimental spectrum of the second positive system can be also attributed to N 2 (C″5Π u) and N 2 (A ′ 5 Σ g +). Other important effects of these perturbations are presented. [ABSTRACT FROM AUTHOR]