l0pt Figure High-throughput quantum chemical calculations, especially those based on Kohn-Sham density functional theory (KS-DFT), have achieved tremendous success in materials discovery. However, due to the notorious self-interaction error, many popular exchange-correlation (XC) functionals suffer from catastrophic failures for molecules with delocalized electronic densities. As an effective but expensive solution to this problem, the optimally tuned range-separation hybrid functional utilizes a system-specific range-separation parameter (ω). [T. Stein, L. Kronik, and R. Baer, J. Am. Chem. Soc. 2009, 131, 2818.] To accelerate the search for optimal ω and makes it practical for the high-throughput materials screening, we developed a stacked ensemble learning approach based on composite molecular descriptors, and assessed its accuracy and efficiency using the LRC-ωPBE functional and a diverse database of over 4,000 molecules that are equally divided into the training and test sets. Compared with the traditional optimal tuning method, our stacked ensemble learning algorithm reached a mean absolute error (MAE) of 0.005 a₀⁻¹ in the values of ω while reducing the time cost by four orders of magnitude. In addition, the predictive power in essential properties such as fundamental and optimal band gaps were not compromised. Given sufficient diversity in the training set, stacked ensemble learning provides a promising alternative scheme to determine range-separation hybrid functional and eventually any system-specific XC functionals.

Over the last few years, we have generated considerable theoretical data on C_nH₂ (n = 5, 7, and 9) ^a,b,c,d and SiC₄H₂ ^e isomers, in view of their astronomical relevance. An overview of these results will be discussed in this meeting. Although the potential target molecules have been theoretically identified, lack of their experimental data, especially the infrared and microwave spectra, is a serious concern. To unambiguously confirm molecules in astrophysical sources, precise laboratory rest frequencies will be required. To the best of our knowledge, all of the new isomers we theoretically identified so far remain elusive in the laboratory itself to date.

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^aThimmakondu, V. S.; Ulusoy, I.; Wilson, A. K.; Karton, A. Theoretical Studies of Two Key Low-Lying Carbenes of C5H2 Missing in the Laboratory, J. Phys. Chem. A, 2019, 123, 6618-6627.
^bThimmakondu, V. S.; Karton. A. Energetic and Spectroscopic Properties of the Low-Lying C7H2 Isomers: A High-Level Ab Initio Perspective, Phys. Chem. Chem. Phys., 2017, 19, 17685-17697.
^cThirumoorthy, K. and Karton, A. and Thimmakondu, V. S. From High-Energy C7H2 Isomers with A Planar Tetracoordinate Carbon Atom to An Experimentally Known Carbene, J. Phys. Chem. A, 2018, 122, 9054-9064.
^dThirumoorthy, K.; Viji, M.; Pandey, A. P.; Netke, T. G.; Sekar, B.; Yadav, G.; Deshpande, S.; Thimmakondu, V. S. Many Unknowns Below or Close to the Experimentally Known Cumulene Carbene - A Case Study of C9H2 Isomers, Chem. Phys. 2019, 527, 110496.
^eJob, N. and Karton, A. and Thirumoorthy, K.; Cooksy, A. L.; Thimmakondu, V. S. Theoretical Studies of SiC4H2 Isomers Delineate Three LowLying Silylidenes Are Missing in the Laboratory, J. Phys. Chem. A 2020, 124, 987-1002.

The increasing number and size of large Line-By-Line (LBL)-databases, routinely exceeding hundreds of millions of lines, calls for spectral synthesis algorithms that can process these large number of lines efficiently. In the special case where the lineshape is identical for every line in the database, the spectrum can be rapidly synthesized by the convolution of a stick-spectrum and the constant lineshape. We propose an integral transform that extends this procedure to databases with unique lineshapes for each line, by generalizing the stick spectrum to a 3D “lineshape distribution function”, which is a function of line position and Gaussian- & Lorentzian width axes. A fast discrete version of the integral transform based on the Fast Fourier Transform (FFT) is formulated for which the computation time scales with c₁N_i + c₂N_νlogN_ν (where N_i is the number of lines, N_ν the number of spectral points, and c₁ and c₂ are small constants), which is much more efficient than the traditional N_i ×N_ν scaling. Furthermore, since the lineshape calculation is no longer a performance bottleneck, lineshapes such as exact Voigt profiles without cutoff can be computed with arbitrary precision. Our procedure can be implemented in existing spectral codes, providing a new level of accuracy and performance for high temperature, high resolution spectroscopy. The procedure was benchmarked against traditional methods within the open source package RADIS: A benchmark HITEMP CO₂ spectrum consisting of 1.8M lines and 200k spectral points took 3.1s, an improvement of  ∼ 300×. The new procedure has since been implemented as the default synthesis method for RADIS. The method's calculations can be easily divided up and are ideally suited for parallel computation. An experimental GPU procedure, also implemented in RADIS, was used to synthesize a CDSD-4000 CO₂ spectrum of 200M lines and 300k spectral points, which took only 150 ms per spectrum regardless of temperature (after an initialization of  ∼ 5 s), or 4·10¹⁴ lines×spectral points/s - an improvement of 5 orders of magnitude compared to the state-of-the-art - with no appreciable loss in accuracy.

Molecular bending spectra can be broadly categorized into three physical cases, depending on the molecular equilibrium configuration: linear, bent, and nonrigid. We have studied the three cases in detail with an extended Hamiltonian (including up to four-body interactions) of the 2D limit of the Vibron Model (2DVM). We have obtained bending band origin predictions within experimental accuracy as well as the corresponding eigenstates for several molecules J. Khalouf-Rivera, F. Pérez-Bernal, M. Carvajal, J. Quant. Spectrosc. Radiat. Transfer (2021) 261, 107436. l0pt Figure The particular spectroscopic signatures characterizing states that straddle the barrier to linearity in nonrigid molecules have been pinpointed with the quantum monodromy and the Birge-Sponer plots. In addition, we propose a characterization of the obtained bending eigenstates making use of the Participation Ratio and the Quantum Fidelity Susceptibility J. Khalouf-Rivera, M. Carvajal, F. Pérez-Bernal, SciPost Physics (2021) submitted (arXiv:2102.12335). The first quantity has already proved useful in determining linearity barriers in molecular bent-to-linear transitions, as well as characterizing transition states in isomerization reactions, e.g., HCN-HNC bond-breaking system J. Khalouf-Rivera, M. Carvajal, L. F. Santos, F. Pérez-Bernal, J. Phys. Chem. A (2019) 123, 44, 9544-9551. Besides, the second quantity allows us to determine in an elegant and basis-independent way the linear or bent character of any excited state. As an application, we have studied, using the above mentioned quantities, the bending spectra of several molecules of interest in different fields: HNC, OCCCO, CH₃NCO, ClCNO, NCNCS, and Si₂C.

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Footnotes:

J. Khalouf-Rivera, F. Pérez-Bernal, M. Carvajal, J. Quant. Spectrosc. Radiat. Transfer (2021) 261, 107436.. J. Khalouf-Rivera, M. Carvajal, F. Pérez-Bernal, SciPost Physics (2021) submitted (arXiv:2102.12335).. J. Khalouf-Rivera, M. Carvajal, L. F. Santos, F. Pérez-Bernal, J. Phys. Chem. A (2019) 123, 44, 9544-9551..

The dispersed fluorescence spectrum from the S₁ origin of PhCCCN is reported. This molecule, which consists of a cyanoacetylene chain fused to an aromatic phenyl ring, is a potential candidate for astronomical detection. The high resolution spectrum was obtained under jet-cooled, gas-phase conditions with a rotational temperature of roughly 2 K, and appears to encode rich information about the vibronic coupling in this species. The experimentally observed peaks are assigned to emissions from the S₁ (Ã\text ¹B₂) origin to a₁ vibrational levels of the ground state, which are dipole allowed and governed by FC factors, and to b₂ vibrational levels of the ground state, made possible by vibronic interactions between S₁ and S₂ (B̃\text ¹A₁) states. The S₀ vibrational structure features a combined Fermi and Darling-Denison resonance near 950 cm⁻¹, which is accurately reproduced by diagonalization of an effective Hamiltonian containing the three a₁ states involved. A KDC Hamiltonian is employed to treat the vibronic b₂ features.

Benchmark scalar-relativistic delta-coupled-cluster calculations of hetero-site double core ionization energies of small molecules containing second-row elements are presented. This study has focused on high-spin triplet components of two-site double core-ionized states, which are single reference in character and consistent with the use of coupled-cluster methods. Contributions to computed double core ionization energies from electron-correlation and basis-set effects as well as corrections to the core-valence separation approximation have been analyzed. Based on systematic convergence of computational results with respect to these effects, delta-coupled-cluster calculations have been shown to be capable of providing accurate double core ionization energies with remaining errors estimated to be below 0.3 eV. The predictions for the double core ionization energies of CF₄, CH₃F, CH₃CF₃, and CH₂FCF₃ are reported. The perspective of these molecules to be used in the experimental studies of two-site double core-ionized states that are involved in x-ray pump/x-ray probe studies of electronic and molecular dynamics following inner shell ionization or excitation is discussed.

[]

X. Zheng, J. Liu, G. Goumy, L. Young, and L. Cheng, J. Phys. Chem. A Comput. 124, 4413-4426 (2020).

l0pt Figure The rotational distributions of the intensities in the low-v vibrational bands are nearly insensitive to the specific forms of the PEFs and DMFs based on the experimental and/or ab initio data. This is not the case for the higher overtone bands. We discuss the problem: Which properties should the model PEF and DMF possess in order to be capable of predicting the intensities of the yet unobserved lines? Arguments are presented that the Born-Oppenheimer PEF and DMF should possess some features following from their properties as functions of the inter-atomic separation as a complex variable. In particular, they must contain branch points associated with the crossings between the ground and excited electronic states. This approach implies that both functions are to be fitted simultaneously to the common data set including both the line positions and the intensities. However, such a problem is very difficult to solve. Here and in Ref. (1), in application to CO, we assume that the PEF is given (2), and we develop an irregular DMF form containing two branch points corresponding to the expected crossings in the complex plane near 0.4 and 2.2 Å. We compare it with an alternative regular function (1) and find that the rotational distribution in the vibrational 7-0 band predicted by the former is very stable with respect to small variations in the data base, as opposed to the regular DMF showing strong instability. The predicted intensities (see figure) are stronger than the HITRAN values calculated by Li et al. (3) with a combined empirical/spline-interpolated ab initio DMF but are close to the ones calculated by us with the purely empirical DMF of Li et al. The irregular function is expected to provide for a reliable prediction of the ro-vibrational line intensities in the 7-0 band. This work was performed in accordance with the state task, state registration No. AAAA-A19-119071190017-7. (1) V. V. Meshkov, A. V. Stolyarov, A. Yu. Ermilov, E. S. Medvedev, V. G. Ushakov, I. E. Gordon, JQSRT (in preparation); (2) JQSRT 217 (2018 ) 262-273. (3) G. Li, I. E. Gordon, L. S. Rothman, Y. Tan, S.-M. Hu, S. Kassi, A. Campargue, E. S. Medvedev, Astrophys. J., Suppl. Ser. 216 (2015) 15.

The rovibrational energy structure of H₂S has been analysed simultaneously for the 20 lowest energy vibrational bands up to J=20. We have used a global analysis rovibrational algebraic approach of which the effective Hamiltonian interaction terms are constructed as products of powers of the angular momentum components and an algebraic realization of the vibrational interactions. The algebraic vibrational terms are obtained as a set of anharmonic Morse ladder operators based on the dynamical algebra U(2), one for every internal coordinate M. Carvajal, R. Lemus, J. Phys. Chem. A 119 (2015) 12823. The parameters of the effective rovibrational Hamiltonian have been determined by a nonlinear least-squares fitting to the available experimental rovibrational energies  K.L. Chubb et al., J. Quant. Spectrosc. Rad. Transf. 218 (2018) 178–186. More than 2500 experimental rovibrational energies, up to J=20 for the 20 lowest vibrational bands, have been fitted with a rms of the order of 10⁻³ cm⁻¹. In total, around 8000 rovibrational energy term values have been calculated.

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Footnotes:

M. Carvajal, R. Lemus, J. Phys. Chem. A 119 (2015) 12823.. K.L. Chubb et al., J. Quant. Spectrosc. Rad. Transf. 218 (2018) 178–186..

r0pt Figure Ionic liquids (ILs) are salts with a melting point below 100 ^°C and are typically composed of asymmetric organic cations and organic or inorganic anions. The modulate effect of the molecular solvent on the properties of ionic liquid thorough understanding of intermolecular interactions between the constituent ionic and molecular species is crucial. One of these effects is ion pairing at low concentrations of the ionic in the molecular solvent. Ion pairing was detected in nonpolar solvents at rather low concentrations of ionic while polar solvents demonstrate the capability of breaking ionic aggregate into separate ions. The ¹H NMR spectra of 10⁻⁵ mole fraction solutions of 1-decyl-3-methyl-imidazolium chloride ionic liquid in water, acetonitrile and dichloromethane have been measured. The chemical shift of the proton at position 2 in the imidazolium ring of 1-decyl-3-methyl-imidazolium is rather different for all three samples, reflecting the shifting equilibrium between the contact pairs and free fully solvated ions. Classical molecular dynamics simulations of the 1-decyl-3-methyl-imidazolium chloride contact ion pair as well as of free ions in three solvents have been conducted, and the quantum mechanics/molecular mechanics methods have been applied to predict NMR chemical shifts for the proton at position 2. By comparing experimental and computational results, we found that the ion pair breaks into free ions in an aqueous solution. Around 23% of the ion pairs were found to form contact pairs in acetonitrile. Ion-pairing breaking into free ions was predicted not to occur in dichloromethane.D. Lengvinaite, V. Klimavicius, V. Balevicius, K. Aidas, J. Phys. Chem. B, 2020, 124, 47, 10776–10786.html:<hr /><h3>Footnotes: D. Lengvinaite, V. Klimavicius, V. Balevicius, K. Aidas, J. Phys. Chem. B, 2020, 124, 47, 10776–10786.

l0pt Figure The K-edge of X-ray absorption near edge structure (XANES) spectra can provide in-situ geometric and electronic information about transition metal catalyst. Due to the complicated characters of the K-edge absorptions, theoretical studies are indispensable to extract such quantitative features. The present work focused on K-edge absorptions of a series of vanadium complex catalysts. The linear-response time-dependent density functional theory (LR-TDDFT) and real-time time-dependent density functional theory (RT-TDDFT) method have been used to calculate the K-edge XANES spectra. The pre-edge region can be well reproduced by LR-TDDFT. The peaks appearing in the pre-edge region were assigned to dipole-allowed transitions of 1s electrons to the 3d4p hybridized orbitals. However, the shoulder peak region located on the K-edge cannot be fully explained by LR-TDDFT calculation due to the complicated multiple excitations and large density rearrangement where the linear response approximation is no longer valid. The greatest strength of RT-TDDFT is that it can produce the entire broad-band spectrum within a single calculation, which happens to serve as a supplement to LR-TDDFT. Recently, Herbert et al. have developed and implement an efficient RT-TDDFT algorithm for computation of X-ray absorption spectra of small molecules. [J. Chem. Phys. 2018, 148, 044117] Herein, we have applied this RT-TDDFT algorithm to probe the absorption features on the raising K-edge. Based on our RT-TDDFT calculations, we have successfully assigned shoulder peaks on the K-edge and characterized the electronic properties of vanadium complexes.

Phosphine is now well established as a biosignature, which has risen to prominence with its recent tentative detection on Venus. To follow up this discovery and related future exoplanet biosignature detections, it is important to spectroscopically detect the presence of phosphorus-containing (P-molecules) atmospheric molecules that could be involved in the chemical networks producing, destroying or reacting with phosphine. Here, we present a high-throughput approach utilising established computational quantum chemistry methods (CQC) to produce a database of approximate infrared spectra for 958 P-molecules that could be spectroscopically detected in planetary atmospheres. We performed the calculations with the strongly performing harmonic ωB97X-D/def2-SVPD model chemistry for all molecules and tested the more sophisticated and time-consuming GVPT2 anharmonic model for 250 smaller molecules. Limitations to our automated approach, particularly for the GVPT2 method, are considered along with pathways to future improvements. Our CQC calculations significantly improve on existing spectroscopic data by providing quantitative intensities, new data in the fingerprint and higher frequency regions, and improved data for fundamental transitions based on the specific chemical environment.

Density functional theory (DFT) has become a popular method for computational work involving larger molecular systems as it provides accuracy that rivals ab initio methods while lowering computational cost. Nevertheless, computational cost is still high for systems greater than ten atoms in size, preventing their application in modeling realistic atmospheric systems at the molecular level. Newer machine learning techniques, however, show promise as cost-effective tools in predicting chemical properties when properly trained. In the interest of furthering chemical machine learning in the field of atmospheric science, we are developing a new training method to be used in the prediction of molecular characteristics for cyclic, nitrogen-based molecules that can undergo tautomerization within brown carbon aerosols. By creating a training dataset made of small molecule DFT calculations that supplement DFT cluster models of brown carbon molecules with hydration shells ranging from one to three waters, we hope to find a significant improvement in accuracy when predicting characteristics of brown carbon compounds while being computationally efficient.

There are long running problems over the precise transition intensities and line positions for ozoneM. A. H. Smith, V. M. Devi & D. C. Benner, "The quest for ozone intensities in the 9-11 μm region: a retrospective", Journal of Quantitative Spectroscopy and Radiative Transfer, 113(11), 825-828 (2012),A. Barbe et al, Özone spectroscopy in the electronic ground state: High-resolution spectra analyses and update of line parameters since 2003", Journal of Quantitative Spectroscopy and Radiative Transfer, 130, 172-190 (2013) In our work, state of the art, first principles quantum mechanical methods are being used to compute high accuracy transition intensities and line positions for the microwave and infrared regions of the spectrum of ozone. In this presentation I will discuss details of our most recent calculations of the ab initio dipole moment surface (DMS) and analysis of experimental ozone line positions using the MARVEL (Measured Active Rotation Vibration Energy Levels) techniqueT. Furtenbacher, A. G. Császár & J. Tennyson, "MARVEL: measured active rotational–vibrational energy levels.", Journal of Molecular Spectroscopy, 245, 115-125 (2007) To improve the quality of our currentO. L. Polyansky, N. F. Zobov, I. I. Mizus , A. A. Kyuberis, L. Lodi & J. Tennyson, "Potential energy surface, dipole moment surface and the intensity calculations for the 10 μm, 5 μm and 3 μm bands of ozone", Journal of Quantitative Spectroscopy and Radiative Transfer, 210, 127-135 (2018)MS we are exploring the electronic structure model, finer grids and larger basis set sizes. The ozone MARVEL project currently involves the analysis of around 70 sources of scientific literature (we anticipate this number to increase). We will use the MARVEL energy levels to fit a new potential energy surface for ozone. We also intend to replace the calculated energy levels with MARVEL energy levels in our line lists. We hope the results of this study will be important for a range of atmospheric studies, such as self-consistency of ozone concentration retrievals in remote sensing techniques, and possible detections of ozone in exo-planetary atmospheres. M. A. H. Smith, V. M. Devi & D. C. Benner, "The quest for ozone intensities in the 9-11 μm region: a retrospective", Journal of Quantitative Spectroscopy and Radiative Transfer, 113(11), 825-828 (2012) A. Barbe et al, Özone spectroscopy in the electronic ground state: High-resolution spectra analyses and update of line parameters since 2003", Journal of Quantitative Spectroscopy and Radiative Transfer, 130, 172-190 (2013). T. Furtenbacher, A. G. Császár & J. Tennyson, "MARVEL: measured active rotational–vibrational energy levels.", Journal of Molecular Spectroscopy, 245, 115-125 (2007). O. L. Polyansky, N. F. Zobov, I. I. Mizus , A. A. Kyuberis, L. Lodi & J. Tennyson, "Potential energy surface, dipole moment surface and the intensity calculations for the 10 μm, 5 μm and 3 μm bands of ozone", Journal of Quantitative Spectroscopy and Radiative Transfer, 210, 127-135 (2018)D

Non-adiabatic couplings (NACs) or derivative couplings (DDRs) between nearly degenerate states can be significant or even divergent. NACs arise from nuclear gradients of electronic wave functions around avoided crossings between adiabatic potential energy curves. Large values of NACs result in strong interaction between corresponding adiabatic states, making the adiabatic approximation invalid. In this work we consider NACs in the case of diatomic open-shell molecules and implement a diabatizaton methodology in the variatonal code DUO.S. N. Yurchenko, L. Lodi, J. Tennyson, A. V. Stolyarov, Comput. Phys. Commun., 2016, 202, 262 - 275; publicly available at https://github.com/ExoMol.he methodology is based on a unitary transformation to the so-called quasi-diabatic states defined to provide smooth (with respect to nuclear coordinates) behaviour of different properties, such as spin-orbit curves, (transition) dipole moment curves etc. As example, rovibronic calculations of the electronic spectra of yttrium oxide, YO,A. N. Smirnov, V. G. Solomonik, S. N. Yurchenko, J. Tennyson, Phys. Chem. Chem. Phys., 2019, 21, 22794-22810.n the quasi-diabatic representation will be presented.

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Footnotes:

S. N. Yurchenko, L. Lodi, J. Tennyson, A. V. Stolyarov, Comput. Phys. Commun., 2016, 202, 262 - 275; publicly available at https://github.com/ExoMol.T A. N. Smirnov, V. G. Solomonik, S. N. Yurchenko, J. Tennyson, Phys. Chem. Chem. Phys., 2019, 21, 22794-22810.i