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Isotopic substitution of muonium for hydrogen provides an unparalleled opportunity to deepen our understanding of quantum mass effects on chemical reactions. A recent topical review in this journal of the thermal and vibrationally state-selected reaction of Mu with H2raises a number of issues that are addressed here. We show that some earlier quantum mechanical calculations of the Mu + H2reaction, which are highlighted in this review, and which have been used to benchmark approximate methods, are in error by as much as 19% in the low-temperature limit. We demonstrate that an approximate treatment of the Born–Oppenheimer diagonal correction that was used in some recent studies is not valid for treating the vibrationally state-selected reaction. We also discuss why vibrationally adiabatic potentials that neglect bend zero-point energy are not a useful analytical tool for understanding reaction rates, and why vibrationally non-adiabatic transitions cannot be understood by considering tunnelling through vibrationally adiabatic potentials. Finally, we present calculations on a hierarchical family of potential energy surfaces to assess the sensitivity of rate constants to the quality of the potential surface. [ABSTRACT FROM AUTHOR]

The advent of very precise measurements of rate coefficients in reactions of muonium (Mu), the lightest hydrogen isotope, with H2in its ground and first vibrational state and of kinetic isotope effects with respect to heavier isotopes has triggered a renewed interests in the field of muonic chemistry. The aim of the present article is to review the most recent results about the dynamics and mechanism of the reaction Mu+H2to shed light on the importance of quantum effects such as tunnelling, the preservation of the zero point energy, and the vibrational adiabaticity. In addition to accurate quantum mechanical (QM) calculations, quasiclassical trajectories (QCT) have been run in order to check the reliability of this method for this isotopic variant. It has been found that the reaction with H2(v=0) is dominated by the high zero point energy (ZPE) of the products and that tunnelling is largely irrelevant. Accordingly, both QCT calculations that preserve the products’ ZPE as well as those based on the Ring Polymer Molecular Dynamics methodology can reproduce the QM rate coefficients. However, when the hydrogen molecule is vibrationally excited, QCT calculations fail completely in the prediction of the huge vibrational enhancement of the reactivity. This failure is attributed to tunnelling, which plays a decisive role breaking the vibrational adiabaticity whenv=1. By means of the analysis of the results, it can be concluded that the tunnelling takes place through the ν1=1 collinear barrier. Somehow, the tunnelling that is missing in the Mu+H2(v=0) reaction is found in Mu+H2(v=1). [ABSTRACT FROM AUTHOR]

Nanoporous materials have the potential to be used as molecular sieves to separate chemical substances in a mixture via selective adsorption and kinetic sieving. The separation of isotopologues is also possible via the so-called quantum sieving effect: the different effective size of isotopologues due to their different Zero Point Energy (ZPE). Here we compare the diffusion coefficients of Hydrogen and Deuterium in (8,0) Single Walled Carbon Nanotubes obtained with quantum dynamics simulations. The diffusion channels obtained present important contributions from resonances connecting the potential wells. These resonances, which are more important for H 2 than for D 2 , increase the low-temperature diffusivity of both isotopologues, but prevent the inverse kinetic isotope effect reported for similar nanostructured systems. [ABSTRACT FROM AUTHOR]

We demonstrate that charge migration can be ‘engineered’ in arbitrary molecular systems if a single localised orbital - that diabatically follows nuclear displacements - is ionised. Specifically, we describe the use of natural bonding orbitals in Complete Active Space Configuration Interaction (CASCI) calculations to form cationic states with localised charge, providing consistently well-defined initial conditions across a zero point energy vibrational ensemble of molecular geometries. In Ehrenfest dynamics simulations following localised ionisation of -electrons in model polyenes (hexatriene and decapentaene) and -electrons in glycine, oscillatory charge migration can be observed for several femtoseconds before dephasing. Including nuclear motion leads to slower dephasing compared to fixed-geometry electron-only dynamics results. For future work, we discuss the possibility of designing laser pulses that would lead to charge migration that is experimentally observable, based on the proposed diabatic orbital approach. [ABSTRACT FROM AUTHOR]

We propose a modified Einstein approximation to describe zero-point energy vibrations in a quantum crystal. Our aim was to develop a computationally cheap tool suitable for lattice structure optimisation. As in the classical Einstein model the representative atom vibrates in an effective potential due to the surrounding atoms of the crystal; the atoms however are not strictly placed at the positions corresponding to the crystal potential energy minima but their positions are described by the quantum mechanical density distributions. The effective potential computed that way is suitable for the application in solid para-hydrogen in contrast to the normal (unmodified) Einstein approximation. We compute the cohesive energy of the para-hydrogen crystal and perform lattice structure optimisation. The hexagonal closed packed is more stable than the fcc closed packed lattice and the lattice constants obtained are in very good agreement with the experimental values. [ABSTRACT FROM AUTHOR]

We report results on the dynamics of tunnelling switching based on a high-resolution spectroscopic investigation of meta-D-phenol in GHz and THz ranges. The pure rotational spectra were recorded in the range of 72–117 GHz and assigned to the localizedsyn- andanti-structures in the ground and the first excited torsional states. Specific torsional states were unambiguously assigned by comparison of the experimental rotational constants with theoretical results from quasiadiabatic channel reaction path Hamiltonian (RPH) calculations. The torsional fundamental νTat ≈ 309 cm−1and the first hot band (2νT– νT) at ≈ 277 cm−1were subsequently assigned in synchrotron based high-resolution Fourier transform infrared (FTIR, THz) spectra. The analyses provided accurate spectroscopic constants of all six states involved. It was found that the 2νTstates are interacting through anharmonic resonances, indicating tunnelling switching as predicted by theory. Furthermore, tunnelling–rotation–vibration transitions were assigned and the tunnelling splitting in 2νTwas determined as 1.72450(17) cm−1. This key result allowed the assignment of two Q branches at 275.21303(9) and 277.67127(9) cm−1to vibration-tunnelling transitions of the (syn←anti) type, hence confirming tunnelling switching dynamics in m-D-phenol. The ground-state energy difference of thesyn- andanti-isotopomers is obtained experimentally asE0(syn)-E0(anti)= (hc)0.82 cm−1in satisfactory agreement with the theoreticalab initiopredictions of (hc)1.5 cm−1given the small absolute values arising from the tiny zero point energy effects. The results are discussed in relation to further fundamental aspects of tunnelling in slightly asymmetric potentials including the effects of the parity violating electroweak interaction in chiral molecules. [ABSTRACT FROM AUTHOR]

Methylketene (CH 3 CHCO) and acrolein (CH 2 CHCHO) have received considerable attention from both theoreticians and experimentalists. In a number of computational studies, it has been identified that both isomers sit alongside their dissociation products C 2 H 4 + CO , at the bottom of the potential energy surface with energies very close to each another. The most striking feature from preliminary calculations is that the relative energy difference between methylketene and trans-acrolein ( Δ E 0.8–3.4 kJ/mol) changes with respect to choice of reference geometry, correlation treatment, and basis set expansion. While the thermodynamic profile favours trans-acrolein as the lowest-lying isomer, it is difficult to estimate the quality of the currently available results, which are determined with a variety of methods. At higher levels of theory together with reducing basis set incompleteness methylketene might be the true minimum on the C 3 H 4 O potential energy surface. In addition to their being almost isoenergetic, much interest in these C 3 H 4 O isomers stems from an astrochemical standpoint. The discovery of both cis and trans-acrolein in the interstellar medium has led to speculation that methylketene may also be present. While thermodynamically methylketene proves to be a good candidate for extra-terrestrial observation, as yet there have been no reports of its detection. To this end, by applying the best correlation treatment that has been used for this species, using by far the most elaborate and accurate basis sets (up to sextuple-zeta quality), we establish the relative ordering and energetic difference between methylketene and trans-acrolein to within chemical accuracy (within 1 kJ/mol). We find the inclusion of zero-point energy, explicit core-correlation, and augmentation with diffuse functions have significant effects in determining the relative energy difference of the isomers, and assignment of the C 3 H 4 O global minimum. [ABSTRACT FROM AUTHOR]

A six-degrees-of-freedom, time-dependent quantum dynamics calculation is employed to study the integral cross sections, full-dimensional cumulative reaction probabilities and full-dimensional rate constants for the isotopic reactions of the OD and OH with CH3 reactions. The full-dimensional cumulative reaction probabilities and full-dimensional rate constants are obtained using the energy and J-K shifting approaches based on the six-degrees-of-freedom calculations. The comparison of integral cross sections shows that the OD + CH3 reaction has a larger energy threshold and a smaller tunnelling effect than the OH + CH3 reaction. The corrected rate constants using the experimental zero-point energy have a very good agreement with the experimental results. The comparison of the rate constants shows that the OD + CH3 has smaller rate constants than the OH + CH3 reaction, which indicates a smaller reactivity due to the isotope substitution. [ABSTRACT FROM AUTHOR]

We reportabinitiocalculations of the molecular structures of the various Cl2O2isomers, transition states, vibrational frequencies and vertical excitation energies, as well as the relative energies of the Cl2O2isomers with respect to 2ClO, ClOO + Cl and OClO + Cl dissociation channels employing up to the CCSD(T)/aug-cc-pVQZ level of theory. Our best theoretical estimate for the dissociation wave numberD0of chlorine-peroxide, dichloride-dioxide ClOOCl relative to 2ClO is 6825 cm−1(including harmonic zero-point energy correction), compared to recent experimental estimates in the range 5700–7000 cm−1, thus favouring the higher values. The chlorine chlorite structure ClOClO is found to be weakly bound by ∼3400 cm−1with respect to 2ClO. The chloryl chloride, chlorine peroxide ClClO2is observed to be stabilised with respect to the chlorine peroxide ClOOCl when large basis sets with diffuse functions are used, and ClClO2is predicted to be abouthc700 cm−1lower in energy than ClOOCl (including harmonic zero-point energy correction). However, ClClO2is not assumed to be significant for the ClO self-reaction due to the high barrier to association. The isomerisations appear also unlikely under stratospheric conditions, as the transition states optimised at CCSD/aug-cc-pVTZ level of theory are found to lie high above the reactants. We also discuss the relation to recent research on parity violation and stereomutation tunnelling in this molecule. [ABSTRACT FROM PUBLISHER]

The paper presents the results of ab initio modelling of the interaction of hydrogen atoms with ferrite/cementite interfaces in steels and thermodynamic assessment of the ability of interfaces to trap hydrogen atoms. Modelling was performed using the density functional theory with generalised gradient approximation (GGA’96), as implemented in WIEN2k package. An Isaichev-type orientation relationship between the two phases was accepted, with a habit plane (101)c∥ (112)α. The supercell contained 64 atoms (56 Fe and 8 C). The calculated formation energies of ferrite/cementite interface were 0.594 J/m2. The calculated trapping energy at cementite interstitial was 0.18 eV, and at the ferrite/cementite interface – 0.30 eV. Considering calculated zero-point energy, the trapping energies at cementite interstitial and ferrite/cementite interface become 0.26 eV and 0.39 eV, respectively. The values are close to other researchers’ data. These results were used to construct a thermodynamic description of ferrite/cementite interface-hydrogen interaction. Absorption calculations using the obtained trapping energy values showed that even thin lamellar ferrite/cementite mixture with an interlamellar spacing smaller than 0.1 μm has noticeable hydrogen trapping ability at a temperature below 400 K. [ABSTRACT FROM AUTHOR]

A novel global potential energy surface for H2S+(X 4A″) based on accurateab initiocalculations is presented. Energies are calculated at the multi-reference configuration interaction level with Davidson correction using aug-cc-pVQZ basis set plus core-polarisation high-exponent d functions. A grid of 4552 points is used for the least-square fitting procedure in the frame of a many-body expansion. The topographical features of the new potential energy surface are here discussed in detail. Such a surface is then employed for dynamic studies of the S(4S) + H2(X 1Σ+g) →SH+(X 3Σ−) + H(2S) reaction using the quasi-classical trajectory method. State specific trajectories are calculated, for both ground and ro-vibrationally excited initial states of H2(X 1Σ+g). Corrections to the zero point energy leakage of the classical calculations are also presented. Calculated reaction cross sections and rate constants are here reported and compared with available literature. [ABSTRACT FROM PUBLISHER]

Zintl (group IV elements) clusters with a special number of valence electrons often exhibit extraordinary structures and chemical bonding patterns, and the stability of them is rationalised by the Wade-Mingos rule. However, this rule is still limited for some Zintl clusters. Here, we find that the potential energy surface of PtSn5 has a single deep funnel, which leads down to the octahedron of the global minimum. The AIMD simulations demonstrate that the octahedral PtSn5 still has a good thermal stability at 1000 K. The molecular orbitals and AdNDP analyses reveal that the 20 valence electrons from five Sn atoms of the PtSn5 fill superatomic shells resulting in an electronic configuration of 1S21P61D102S2, and the electronic structure of the Pt atom is d10, well following the magic number of the Jellium model. The result is further confirmed by the density of states and the localised orbital locator. After the CO molecule is adsorbed on the PtSn5, it prefers to be located at the Pt atom. Due to the charge-transfer effect, the C–O bond length and stretching frequency appear to change significantly. Our work opens up the study on the Jellium model for Zintl clusters. [ABSTRACT FROM AUTHOR]

In this study, we theoretically investigate the interactions of hydrogen bonding in excited states and the mechanism of intramolecular proton transfer (ESIPT) for the novel compound 3-benzothiazol-2-yl-2-hydroxy-5-methyl-benzaldehyde-oxime (HBT-phos) theoretically. Herein, the effects of atomic electronegativity of chalcogen (O, S and Se) are also focused. Firstly, employing quantum theory of atoms-in-molecules (QTAIM) method, we confirm intrinsic quality of O-H···N within HBT-phos derivatives (HBT-phos-O, HBT-phos-S and HBT-phos-Se). By astutely examining changes in geometries and infrared (IR) spectra, we elegantly demonstrate O-H···N is remarkably strengthened in S1 state, indicating a pronounced tendency towards ESIPT. Computational hydrogen bonding energies further magnificently support this conclusion. Probing into photo-induced excitation, we discover heightened electronic densities surrounding N atom that play pivotal roles in attracting protons, thereby facilitating ESIPT. Ultimately, upon comparing potential energy curves in excited states, we deduce ESIPT process is expected to be exceptionally rapid with low atomic electronegativity (O → S → Se), which elucidates why detecting fluorescence peak of HBT-phos-S itself proves challenging in experiment. This study not only fills the void in understanding ESIPT mechanism within HBT-phos derivatives and authentically reveals its unambiguous mechanism, but also presents atomic-electronegativity-dependent ESIPT behaviour and the explanation for fluorescence quenching observed in prior experiment. [ABSTRACT FROM AUTHOR]

Developing highly efficient catalysts for carbon dioxide reduction reaction (CO2RR) is significant in producing useful chemicals and alleviating environmental pollution. Herein, a series of two-dimensional TM-PTC (TM = Sc-Zn, PTC = 1,2,3,4,5,6,7,8,9,10,11,12-perthiolated coronene) frameworks as CO2RR electrocatalysts are investigated based on the density functional method. The calculated results showed that the main product is CH4 for TM-PTC (TM = Sc-Mn). However, these catalysts can be poisoned by water molecules due to its strong adsorption on the catalyst surface. To solve the problem, we modified these catalysts by adding axial oxygen (TM-O-PTC). Our study showed that TM-O-PTC (TM = V, Cr) are good catalysts for CO2RR to produce CH4. We expected that this work could provide a useful strategy for developing high-performance CO2RR electrocatalysts. [ABSTRACT FROM AUTHOR]

The Haber-Bosch process, commonly used for ammonia synthesis, can be replaced by the electrochemical nitrogen reduction reaction (NRR), which offers a more environmentally conscious and sustainable approach. However, developing electrocatalysts with low potential, high selectivity, and fast reaction kinetics remains challenging. In the current work, we have theoretically calculated the potential of V@B5N3 and V@B7N5 (V = Vanadium) material which serves as an electrocatalyst for NRR using density functional theory (DFT) simulations. Our results reveal that the monolayer of V@B7N5 exhibits remarkable catalytic activity and effectively suppresses the hydrogen evolution reaction at an overpotential of 0.13 V. Furthermore, the potential-determining step involving the conversion of *N2-*N2H demonstrates a favourable energy barrier, indicating facile kinetics. The V@B7N5 monolayer shows promising selectivity, with an anticipated ammonia production efficiency of 99%. These findings open avenues for employing single-atom catalysts under ambient conditions to synthesise ammonia, offering a potential solution for sustainable ammonia production. [ABSTRACT FROM AUTHOR]

Explicit hydration studies of ions require accurate estimation of interaction energies. This work explores the explicit hydration of the ammonium ion (NH4+) employing Møller–Plesset second order (MP2) perturbation theory, an accurate yet relatively less expensive correlated method. Several initial geometries of NH4+(H2O)n(n= 4 to 13) clusters are subjected to MP2 level geometry optimisation with correlation consistent aug-cc-pVDZ (aVDZ) basis set. For large clusters (viz.n> 8), molecular tailoring approach (MTA) is used for single point energy evaluation at MP2/aVTZ level for the estimation of MP2 level binding energies (BEs) at complete basis set (CBS) limit. The minimal nature of the clusters upton≤ 8 is confirmed by performing vibrational frequency calculations at MP2/aVDZ level of theory, whereas for larger clusters (9 ≤n≤ 13) such calculations are effected via grafted MTA (GMTA) method. The zero point energy (ZPE) corrections are done for all the isomers lying within 1 kcal/mol of the lowest energy one. The resulting frequencies in N–H region (2900–3500 cm−1) and in O–H stretching region (3300–3900 cm−1) are in found to be in excellent agreement with the available experimental findings for 4 ≤n≤ 13. Furthermore, GMTA is also applied for calculating the BEs of these clusters at coupled cluster singles and doubles with perturbative triples (CCSD(T)) level of theory with aVDZ basis set. This work thus represents an art of the possible on contemporary multi-core computers for studying explicit molecular hydration at correlated level theories. [ABSTRACT FROM AUTHOR]

Holes are commonly associated with high-dimensional, non-parametric representations of potential energy surfaces (PESs). These are regions of the PES with large negative values. Typically these occur at highly distorted geometries of (in reality) high energy. Finding holes in a systematic and efficient way is thus an important aspect in the development of PESs. Here we propose using diffusion Monte Carlo (DMC) with artificially reduced masses to automatically and robustly sample large regions of a fit, including those at very high energies. We demonstrate this for a recent PES of methane that was obtained using energies and gradients. The DMC software with the methane potential is available for download. [ABSTRACT FROM AUTHOR]

The nitro groups are introduced into 4-(1H-1,2,4-triazol-1-yl) pyrimidine to substitute the hydrogen atoms successively, through which a total of 31 derivatives are constructed to look for high energy density compounds (HEDCs). To probe the thermal stabilities, the heats of formation (HOFs) are calculated by using the G3MP2 method. To explore the dynamic stability, the natural bond orbital (NBO) analysis is performed to confirm the trigger bond, for which the bond dissociation energy is calculated using the B3PW91 method with the 6-311 + G(d,p) basis set. Furthermore, the detonation velocity (D), the detonation pressure (P), the molecular density (ρ), the explosive heat (Q), and character height (H50) are also predicted. Based on our calculation, it is confirmed that D1, D2, D5, and E are potential high-energy-density compounds with sufficient stability and excellent detonation performance. [ABSTRACT FROM AUTHOR]

Conformational states, barriers to internal rotation, 2D potential energy surfaces, kinetic coefficients and dipole moment components of the resorcinol molecule are calculated at the MP2/CBS(T,Q), MP2/Aug-cc-pVTZ, MP2/dAug-cc-pVTZ and B3LYP/Aug-cc-pVTZ levels of theory. Using the calculated data sets, the energies and wave functions of stationary torsional states were determined for the first time using a numerical solution of the vibrational Schrödinger equation of limited dimensionality. This made it possible to establish the values of the tunneling splitting of the ground vibrational and a number of excited torsional states of the energetically most preferred conformer of the molecule, belonging to the point symmetry group CS. The 100 lowest torsional states of the resorcinol molecule are classified according to the symmetry species of the molecular symmetry group C2V(M). The torsional IR spectra of three conformers of the molecule were simulated at different temperatures. The calculated values of the frequency of the most intense torsional vibration in the most stable conformer of the molecule (316 cm−1) is in good agreement with the experimental value of the frequency of this vibrations (318 cm−1), obtained in [W.G. Fateley, J.Phys.Chem., 79 (1975) 199–204.]. [ABSTRACT FROM AUTHOR]

To obtain novel high-energy density compounds with superior detonation properties and sensitivity to those of RDX, a series of furazan/tetrazole/tetrazine-tetrazole-N = N-tetrazole-furazan/tetrazole/tetrazine and furazan/tetrazole/tetrazine-tetrazole-NHNH-tetrazole-furazan/tetrazole/tetrazine-based energetic materials were designed. Their structures were optimised by Gaussian software with density functional theory at B3LYP/6-311G (d,p) level. Based on the optimised structures, their frontier orbital energies, heats of formation and detonation properties were calculated. The results indicate that the compound with tetrazole-tetrazole-N = N-tetrazole-tetrazole parent structure and -N3 energetic group has the highest value of heat of formation while the compound with tetrazine-tetrazole-NHNH-tetrazole-tetrazine parent structure and -C(NO2)3 energetic group possess the highest values of detonation velocity and detonation pressure. Most of the designed compounds have higher values of h50 than those of RDX (28 cm) and HMX (32 cm), and series E had higher average values of impact sensitivity. Finally, taking detonation properties and impact sensitivity into consideration, six compounds (A2, A8, C8, D2, F7 and F8) were selected as potential candidate materials for high-energy density materials, and their electronic and physicochemical properties were investigated. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT)-based calculations were performed for 36, 72 and 108 water molecules forming twisted ribbon-like nano-ring structures of diameter 1.54, 3.10 and 4.56 nm, respectively. We explore the electric field evolution of the structure, energetics and stability of the water nano-ring structures that are essentially symmetric and non-polar, or less-symmetric and polar, hydrogen-bonded clusters. The results suggest that for fields applied perpendicular to the ring, larger rings are more susceptible to the field influence, while fields applied parallel to the ring quickly transform the ring structures into arbitrary ones, regardless of the ring size. Infrared and Raman spectral analysis of local modes demonstrate the typical vibrational response of water molecules to various H-bonding environments and applied fields, providing a spectroscopic signature that can be used to identify the manifestation of such H-bonded ring networks. Our study underscores the implications of the nano-sized ring water clusters, which have the potential to be exploited in a variety of device applications. [ABSTRACT FROM AUTHOR]

Phosphorus is an element essential to sustaining life, and its compounds are widely involved in the biogeochemical cycles on Earth. Although a handful of phosphorus-bearing species, e.g. PO, have thus far been detected in diatomic form in circumstellar envelopes, their formation processes are still poorly understood. In this study, we examine the structure and spectroscopy of the [Si, O, P] system. Based on high-level ab initio electronic structure calculations with the MRCI + Q and CCSD(T) methods, four isomers could be formed and the most stable form predicted to be bent at all theoretical levels. The doublet and quartet potential energy surfaces (PESs) of P(2D)/P(4S) + SiO(1Σ) reactions are explored, and the spectroscopic parameters of the key isomers involved in the reaction are calculated. The present study highlights the possibility of PO formation via photodissociation of bent-SiOP. The results are expected to aid in the spectroscopic detection of these triatomic phosphorous-bearing species and provide insights into similar reactions of isoelectronic systems. [ABSTRACT FROM AUTHOR]

New computational and experimental studies have been carried out for the MgCCH radical in its X2Σ+ state. Coupled cluster theory [CCSD(T)], was used in conjunction with post-CCSD(T) and scalar relativistic additive corrections to compute vibrational quartic force fields for MgCCH and its cation. From the quartic force fields, higher-order spectroscopic properties, including rotational constants, were obtained. In tandem, the five lowest energy rotational transitions for MgCCH, N = 1→0 through N = 5→4, were measured for the first time using Fourier transform microwave/millimetre wave methods in the frequency range 9–50 GHz. The radical was created in the Discharge Assisted Laser Ablation Source (DALAS) developed in the Ziurys group. A combined fit of these data with previous millimetre direct absorption measurements have yielded the most accurate rotational constants for MgCCH to date. The computed principle rotational constant lies within −1.51 to +1.65 MHz of the experimental one, validating the computational approach. High-level theory was then applied to produce rovibrational spectroscopic constants for MgCCH+, including a rotational constant of B0 = 5354.5–5359.5 MHz. These new predictions will further the experimental study of MgCCH+, and aid in the low-temperature characterisation of MgCCH in the interstellar medium. [ABSTRACT FROM AUTHOR]

We report a new permutationally invariant polynomial (PIP) fit to CCSD(T)/aug-cc-pVTZ energies for $ \hbox {CH}_5^{+} $ CH 5 + that provides fast analytical forces, unlike the original PIP fit of 2006. The new fit, which is slightly more precise than the previous one, is used in classical molecular dynamics, diffusion Monte Carlo and path integral molecular dynamics calculations of CH and HH radial distribution functions (RDFs) at temperatures of 10 and $ 50\,\hbox {K} $ 50 K . Corresponding RDFs are also reported for $ \hbox {CD}_{3}\hbox {H}_{2}^{+} $ CD 3 H 2 + . The well-known quantum localisation of the H atoms to the diatomic site from previous and present DMC calculations is maintained at $ 10\,\hbox {K} $ 10 K but shows additional delocalisation to the ' $ \hbox {CH}_{3}^{+} $ CH 3 + ' site at $ 50\,\hbox {K} $ 50 K . [ABSTRACT FROM AUTHOR]

Time-dependent density functional theory (TDDFT) offers a tractable means to predict electronic excitation spectra of ices if sufficient care is taken in selecting the DFT functional, basis sets, and the number of excited states. In this work 48 functionals in conjunction with various correlation consistent basis sets were benchmarked for predicting electronic spectra. A training set of important astromolecules was chosen: carbon monoxide, water, hydrogen cyanide, ammonia, methanol, and the hydroxyl radical. Vertical excitation energies between the ground and first excited state and some higher excited states were calculated using multireference configuration interaction calculations to serve as benchmarks for gas phase TDDFT calculations. When used in conjunction with aug-cc-pVDZ quality correlation consistent basis sets, two functionals, MPW1K and ωB97, emerged as very good choices to model electronic excitations for most of the molecules in the training set. Cluster calculation predictions for the UV spectra of amorphous ices of water and ammonia were performed at the TDDFT-MPW1K/AVDZ and TDDFT-ωB97/AVDZ levels and then compared against experimental data. The calculations qualitatively reproduce the shapes of the spectra for the most part and quantitatively predict the position of the critical first absorption peak. [ABSTRACT FROM AUTHOR]

To facilitate the data analysis of current and future high-resolution space telescope missions, we adopt 'Best Theory + Reliable High-resolution Experiment' (BTRHE) strategy to develop highly accurate infrared line lists for nitrous oxide (N2O). The 'Ames-1' potential energy surface (PES) is a CCSD(T)/aug-cc-pVQZ PES refined using selected HITRAN experimental data, with σrms = 0.02–0.03 cm-1 for five isotopologues. The 'Ames-1' dipole moment surface (DMS) is fitted from CCSD(T)/aug-cc-pV(T,Q,5)Z dipoles extrapolated to one electron basis set limit. Using the Ames-1 PES and DMS, Ames-296K line lists are computed in the full range of 0–15,000 cm-1 for 12 N2O isotopologues, with S296K ≥ 10−31 cm-1/molecule.cm-2. The reliability and consistency of Ames-296K intensity predictions (SAmes) are demonstrated through comparisons with HITRAN (SHITRAN), NOSL-296 (SNOSL), recent observed intensities (Sobs) and Effective Dipole Model (EDM) intensities (SEDM). Agreements and discrepancies are discussed, along with preliminary uncertainty estimate for SAmes. The SAmes provides a good constraint to prevent substantial errors in intensity predictions (e.g. for weak bands and minor isotopologues) and can be further improved. Ames-296K and NOSL-296 may complement each other to provide improved input for future database updates, combining the strengths of EH/EDM and BTRHE approaches. Data available at and https://doi.org/10.48667/9kmk-0334. [ABSTRACT FROM AUTHOR]