Correlation-consistent Polarized Valence Triple-zeta Research Articles

Photoionization of aziridine: Nonadiabatic dynamics of the first six low-lying electronic states of the aziridine radical cation.

In this article, the theoretical photoionization spectroscopy of the aziridine (C2H5N) molecule is investigated. To start with, we have optimized the geometry of this molecule at the neutral electronic ground state at the density functional theory/augmented correlation-consistent polarized valence triple zeta level of theory using the G09 program. The electronic structure calculations were restricted to the first six low-lying electronic states in order to account for the experimental photoelectron spectrum of the C2H5N molecule. The first six low-lying electronic states (X̃2A', Ã2A', B̃2A″, C̃2A″, D̃2A', and Ẽ2A') of the potential energy surfaces (PESs) are calculated by both equation of motion-ionization potential-coupled cluster singles and doubles and multi-configuration quasi-degenerate perturbation theory abinitio quantum chemistry methods along the dimensionless normal displacement coordinates in which multiple conical intersections were established among the considered electronic states. A (6 × 6) model vibronic Hamiltonian is constructed on a diabatic electronic basis, using the symmetry selection rules and Taylor series expansion. The Cs symmetry point group of the aziridine molecule leads to electronic states symmetry of either A' or A″, and these states are close in energy, due to which the same symmetry electronic states avoid each other. To get a smooth diabatic PES, a fourfold diabatization scheme is used, which is implemented in the General Atomic and Molecular Electronic Structure Systems suite of programs. All the parameters used in the diabatic vibronic coupling model Hamiltonian are calculated in terms of the normal modes of vibrational coordinates. Finally, the vibronic model Hamiltonian constructed for the coupled six electronic states is used to solve both time-independent and time-dependent Schrödinger equations using the multi-configuration time-dependent Hartree program module to obtain the dynamical observables. The theoretical vibronic band structure is found to be in good accord with the available experimental results.

Global and Full-Dimensional Potential Energy Surfaces of the N2 + O2 Reaction for Hyperthermal Collisions.

The energy transfer, dissociations, and chemical reactions between O2 and N2 play an important role in the re-entry process of aircraft and many atmospheric, combustion, and plasma processes. Recently, Varga et al. (J. Chem. Phys., 2016, 144, 024310) developed a full-dimensional high-precision potential energy surface (PES) of the ground triplet electronic state for the O2 and N2 system based on ca. 55,000 data points, whose energies were calculated by multi-state complete-active-space second-order perturbation theory/minimally augmented correlation-consistent polarized valence triple-zeta electronic structure calculations plus dynamically scaled external correlation. The fitting function adopted the many-body expansion form with the four-body interactions fitted by the permutationally invariant polynomial in terms of bond-order functions of the six interatomic distances (MB-PIP). In this work, we refit the PES of the N2O2 system by two methods based on the same data set that was used by Varga et al. The first uses the permutation invariant polynomial-neural network (PIP-NN) method to fit the entire energy of the 55,000 data points. In the second approach, the PIP-NN method is used to fit only the four-body interaction component, a similar treatment in the MB-PIP method, and the resulting PES is named MB-PIP-NN. Then, the performances of these new PESs and the MB-PIP PES are comprehensively and systematically compared, such as comparisons of various scans, properties of stationary points, and dynamics simulations. Possible improvements for the PES of N2O2 are suggested. A more reliable PES of the system can be constructed in terms of data sampling range, electronic structure calculation level, and fitting method for high-temperature calculation and simulation in the future.

Collisional excitation of NH by H2: Potential energy surface and scattering calculations.

Collisional data for the excitation of NH by H2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH-H2 van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations [RCCSD(T)-F12a] with the augmented correlation-consistent polarized valence triple zeta basis set. The PES was represented by an angular expansion in terms of coupled spherical harmonics. The global minimum corresponds to the linear structure with a well depth De = 149.10 cm-1. The calculated dissociation energy D0 is found to be 30.55 and 22.11 cm-1 for ortho-H2 and para-H2 complexes, respectively. These results are in agreement with the experimental values. Then, we perform quantum close-coupling calculations of the fine structure resolved excitation cross sections of NH induced by collisions with ortho-H2 and para-H2 for collisional energies up to 500 cm-1. We find strong differences between collisions induced by ortho-H2 and para-H2. Propensity rules are discussed. The cross sections are larger for fine structure conserving transitions than for fine structure changing ones, as predicted by theory. These new results should help in interpreting NH interstellar spectra and better constrain the abundance of NH in interstellar molecular clouds.

The structural, vibrational and electronic properties of the triel-bonded complexes of boron trifluoride with some cyclic ethers – An ab initio study

The properties (molecular structures, vibrational spectra and electronic distributions) of the triel-bonded complexes of boron trifluoride with the cyclic ethers oxirane, oxetane, tetrahydrofuran and tetrahydropyran have been computed. The calculations were carried out ab initio using the Gaussian 16 program at the second order level of Møller-Plesset perturbation theory (MP2) and with Dunning’s augmented correlation-consistent polarized valence triple-zeta basis sets (aug-cc-pVTZ). The results indicate that, with the exception of the oxirane complex, the interaction energies, the bond length and angle changes and the perturbations of the BF3 fragment vibrational wavenumbers are remarkably invariant to the ring size of the base. The unique position of the oxirane complex is associated with the extent of ring strain in the monomer relative to those of the other three bases.

A new four-dimensional ab initio potential energy surface and rovibrational spectra for the C2H2–Ar complex

ABSTRACT We report a new four-dimensional potential energy surface for the C2H2–Ar complex involving the and normal modes for the symmetric stretching and asymmetric stretching vibration of the C2H2 molecule. The potential was constructed at the coupled-cluster singles and doubles with noniterative inclusion of connected triples [CCSD(T)]-F12 level with augmented correlation-consistent polarized-valence triple-zeta (aug-cc-pVTZ) basis set supplemented with midpoint bond functions (3s3p2d1f1g). Two vibrationally averaged potentials with C2H2 at the vibrational ground and excited states were obtained by the integration of the potential over the and coordinates. It was found that each averaged potential has two equivalent T-shaped global minima, two equivalent local minima as well as three saddle points among them. The global minima are located at = 3.97 Å, = 61.4°/118.6° with a well depth of 125.47 cm−1. The rovibrational energy levels and bound states were calculated adopting radial discrete variable representation (DVR)/angular finite basis representation (FBR) and the Lanczos algorithm. The calculated band origin for C2H2–Ar is red-shift by −0.9937cm−1 in the region of C2H2, which reproduces the observed value of −1.0749cm−1 successfully. Furthermore, the calculated microwave and infrared transition frequencies are in well good agreement with the experiment data.

Collisional excitation of interstellar PO(X2Π) by He: new ab initio potential energy surfaces and scattering calculations.

We present the first ab initio potential energy surfaces (PESs) for the PO(X2Π)-He van der Waals system. The PESs were obtained using the open-shell partially spin-restricted coupled cluster approach with single, double and perturbative triple excitations [UCCSD(T)]. The augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) basis set was employed supplemented by mid-bond functions. Integral and differential cross sections for the rotational excitation in PO-He collisions were calculated using the new PES and compared with results in similar systems. Finally, our work presents the first hyperfine-resolved cross sections for this system that are needed for accurate modelling in astrophysical environments.

The structures and vibrational spectra of the methyl halide dimers. An ab initio study

Four homodimers and six heterodimers of the methyl halides, CH3X (X = F, Cl, Br, I), have been examined by means of ab initio calculations at the second order level of Møller-Plesset perturbation theory and with an augmented correlation-consistent polarized valence triple-zeta basis set. Three families of dimers have been found. All four homodimers optimize as both cyclic doubly hydrogen-bonded CH … X species and as non-hydrogen-bonded van der Waals C … X aggregates. All six heterodimers also optimize with cyclic doubly bonded structures; in addition, three of them are non-hydrogen-bonded with C … X linkages, and the three containing CH3I are singly bonded, with a CH … I interaction. The properties normally regarded as distinguishing features separating red-shifted from blue-shifted hydrogen bond interactions (CH bond length changes, shifts of the CH stretching wavenumbers and infrared CH stretching band intensity ratios) have been determined, and their variations with respect to the structures of the adducts have been rationalized.

Potential energy surface of triplet N2O2

We present a global ground-state triplet potential energy surface for the N2O2 system that is suitable for treating high-energy vibrational-rotational energy transfer and collision-induced dissociation. The surface is based on multi-state complete-active-space second-order perturbation theory/minimally augmented correlation-consistent polarized valence triple-zeta electronic structure calculations plus dynamically scaled external correlation. In the multireference calculations, the active space has 14 electrons in 12 orbitals. The calculations cover nine arrangements corresponding to dissociative diatom-diatom collisions of N2, O2, and nitric oxide (NO), the interaction of a triatomic molecule (N2O and NO2) with the fourth atom, and the interaction of a diatomic molecule with a single atom (i.e., the triatomic subsystems). The global ground-state potential energy surface was obtained by fitting the many-body interaction to 54 889 electronic structure data points with a fitting function that is a permutationally invariant polynomial in terms of bond-order functions of the six interatomic distances.

Communication: An accurate full 15 dimensional permutationally invariant potential energy surface for the OH + CH4 → H2O + CH3 reaction.

A globally accurate full-dimensional potential energy surface (PES) for the OH + CH4 → H2O + CH3 reaction is developed using the permutation invariant polynomial-neural network approach based on ∼135,000 points at the level of correlated coupled cluster singles, doubles, and perturbative triples level with the augmented correlation consistent polarized valence triple-zeta basis set. The total root mean square fitting error is only 3.9 meV or 0.09 kcal/mol. This PES is shown to reproduce energies, geometries, and harmonic frequencies of stationary points along the reaction path. Kinetic and dynamical calculations on the PES indicated a good agreement with the available experimental data.

Is the reaction between formic acid and protonated aminomethanol a possible source of glycine precursors in the interstellar medium?

Context. One of the most interesting questions in interstellar chemistry concerns whether we can detect the basic building blocks of proteins in astronomical sources. In ascertaining whether amino acids could be possible interstellar molecules, a crucial point is how they could be synthesized in the interstellar medium.Aims. We do a theoretical study of the ion-molecule reaction involving protonated aminomethanol and formic acid to establish its viability in space. This ion-molecule reaction has been proposed by other authors as a possible way to produce glycine in the interstellar medium.Methods. The relevant stationary points on the potential energy surface of the reaction between protonated aminomethanol and formic acid have been theoretically studied by using ab initio methods. The second-order Moller-Plesset level was employed, in conjunction with the correlation-consistent polarized valence triple-zeta (cc-pVTZ) basis set. In addition, the electronic energies were refined by means of single-point calculations at the CCSD(T) level (coupled cluster single and double excitation model augmented with a non-iterative treatment of triple excitations) on the MP2/cc-pVTZ geometries with the aug-cc-pVTZ basis set.Results. Formation of protonated glycine is an exothermic process; however, the process presents a net activation barrier that makes this reaction unfeasible under interstellar conditions.Conclusions. The reaction of protonated aminomethanol with formic acid does not seem to be a plausible source of interstellar glycine. This particular case is a clear example that a detailed study of the potential energy surface is needed to establish the relevance of a process in the interstellar medium.

The molecular complexes of boron trifluoride with nitrosyl fluoride and nitrosyl chloride. Ion-pair formation

The electron donor–acceptor complexes formed between boron trifluoride and the nitrosyl halides ONF and ONCl have been studied by means of ab initio molecular orbital theory at the second order level of Møller–Plesset perturbation theory and with Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. The focus of the calculations was on the structures, interaction energies and vibrational spectra of the complexes. A variety of trial structures were examined, with electron donation occurring from the oxygen, nitrogen and halogen atoms, in an attempt to establish the most favorable site for interaction. In addition, a number of rotational isomers for each adduct were investigated. It was found that, in both cases, the halogen atom was the preferred donor atom. Several of the optimized structures suggested that the formation of ion pairs would lead to stable complexes, and four separate ion-pair structures were included among the possible associated species for each combination of interacting molecules. In those cases the computed spectra were more consistent with those of the NO+ and BF4− or BClF3− ions than of the corresponding neutral components. The relative stabilities of the two families of complexes have been rationalized and some differences between the properties of the ONF and ONCl adducts have been observed and explained.

The evolution of the structural, vibrational and electronic properties of the cyclic ethers – on ring size. An ab initio study

The molecular structures, vibrational spectra and atomic charges of the alicyclic ethers containing from two to five carbon atoms have been determined by means of ab initio calculations, at the level of second order Møller–Plesset perturbation theory and using Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. Two isomers of the oxetane, tetrahydrofuran and tetrahydropyran molecules have been identified and their relative energies determined. Structural properties, such as the COC bond angles and the CH bond lengths, are found to increase steadily with increasing ring size and with decreasing ionization energy. The mean CH2 stretching and bending wavenumbers exhibit the reverse behaviour, while the mean wavenumbers of the CH2 wagging and twisting modes follow the same trend as the structural features. The ring mode wavenumbers vary in a less regular way. The charges of the oxygen, α-carbon and axial and equatorial α- and β-hydrogen atoms also do not show systematic dependences on ring size or ionization energy. The trends in the values of these properties have been rationalized.

Communication: An accurate global potential energy surface for the OH + CO → H + CO2 reaction using neural networks

We report a new global potential energy surface of the HOCO system based on the F12 correction of unrestricted coupled-cluster with singles doubles and approximative triples using the augmented correlation-consistent polarized valence triple-zeta basis set (UCCSD(T)-F12/AVTZ), fitted by using the neural networks. Quantum dynamics calculations confirmed the satisfactory convergence of surface with respect to the number of data points and fitting process. It is found that the total reaction probabilities and complex-formation probabilities obtained on the present surface differ considerably with those obtained on the potential energy surface recently reported by Li et al. [J. Chem. Phys. 136, 041103 (2012)]. Various comparisons revealed that the present surface is substantially more accurate than that surface, representing the best available potential energy surface for this benchmark complex-forming four-atom system.

An ab initio study of the properties of some lithium-bonded complexes – Comparison with their hydrogen-bonded analogues

Ab initio calculations have been carried out on a series of complexes formed between lithium fluoride, chloride and bromide on the one hand, and ammonia, water, phosphine and hydrogen sulfide on the other. The calculations were performed using the Gaussian-09 program, at the second order level of Møller–Plesset perturbation theory and with Dunning’s augmented correlation-consistent polarized valence triple-zeta basis set. The properties studied were the molecular structures, interaction energies and vibrational spectra. The results have been compared with those for an analogous set of complexes formed between the three acids hydrogen fluoride, chloride and bromide, and the same four Lewis bases. Common features between the properties of both sets of complexes have been highlighted, and the differences rationalized.

Relativistic diffusion Monte Carlo method: Zeroth-order regular approximation-diffusion Monte Carlo method in a spin-free formalism

A diffusion Monte Carlo (DMC) method for the relativistic zeroth-order regular approximation (ZORA) is proposed. In this scheme, a novel approximate Green's function is derived for the spin-free ZORA Hamiltonian. Several numerical tests on atoms and small molecules showed that by combining with the relativistic cusp-correction scheme, the present approach can include both relativistic and electron-correlation effects simultaneously. The correlation energies recovered by the ZORA-DMC method are comparable with the nonrelativistic DMC results and superior to the coupled cluster singles and doubles with perturbative triples correction results when the correlation-consistent polarized valence triple-zeta Douglas-Kroll basis set is used. For the heavier CuH molecule, the ZORA-DMC estimation of its dissociation energy agrees with the experimental value within the error bar.

New vibrational assignments for the ν 1 to ν 17 vibrational modes of aziridine and first analysis of the high-resolution infrared spectrum of aziridine between 720 and 1050 cm−1

Fourier transform spectra of aziridine (C2H4NH) were recorded at high resolution (0.002 or 0.003 cm−1) in the 600–1750 and 1750–4000 cm−1 regions, using a Bruker IFS125HR (an upgraded version of the IFS120HR) spectrometer, located at the LISA facility in Creteil. In parallel, the harmonic force field of aziridine was evaluated analytically at the optimized geometry with second-order Møller–Plesset perturbation theory (MP2) together with the correlation-consistent polarized valence triple zeta basis sets cc-pVTZ. These ab initio predictions were used to perform consistent vibrational assignments for the ν 1 to ν 17 fundamental bands of aziridine observed in the infrared spectra recorded during this study. Finally, a first detailed rotational assignment was performed for two B-type bands located at 772.3571 cm−1 (ν 10, CH2 rock) and 997.1592 cm−1 (ν 8, NH bend) and for an A-type band located at 904.0429 cm−1 (ν 17, ring deform). We noticed that the ν 10 band is weakly perturbed, presumably because the v 10 = 1 rotational levels are coupled with those of the v 18 = 1 dark band located around 817 cm−1 through B-type and C-type Coriolis resonances.

Ab initio potential energy surfaces for NH(Σ3−)–NH(Σ3−) with analytical long range

We present four-dimensional ab initio potential energy surfaces for the three different spin states of the NH(Σ3−)–NH(Σ3−) complex. The potentials are partially based on the work of Dhont et al. [J. Chem. Phys. 123, 184302 (2005)]. The surface for the quintet state is obtained at the RCCSD(T)/augmented correlation-consistent polarized valence triple-zeta (aug-cc-pVTZ) level of theory and the energy differences with the singlet and triplet states are calculated at the complete active space with nth-order perturbation theory/aug-cc-pVTZ (n=2,3) level of theory. The ab initio potentials are fitted to coupled spherical harmonics in the angular coordinates, and the long range is further expanded as a power series in 1/R. The RCCSD(T) potential is corrected for a size-consistency error of about 0.5×10−6 Eh prior to fitting. The long-range coefficients obtained from the fit are found to be in good agreement with first and second-order perturbation theory calculations.

Quantum dynamics of vibrational excitations and vibrational charge transfer processes in H+ + O2 collisions at collision energy 23 eV

Quantum mechanical study of vibrational state-resolved differential cross sections and transition probabilities for both the elastic/inelastic and the charge transfer processes have been carried out in the H+ + O2 collisions at the experimental collision energy of 23 eV. The quantum dynamics has been performed within the vibrational close-coupling rotational infinite-order sudden approximation framework employing our newly obtained quasi-diabatic potential energy surfaces corresponding to the ground and the first excited electronic states which have been computed using ab initio procedures and Dunning’s correlation consistent-polarized valence triple zeta basis set at the multireference configuration interaction level of accuracy. The present theoretical results for elastic/inelastic processes provide an overall agreement with the available state-selected experimental data, whereas the results for the charge transfer channel show some variance in comparison with those of experiments and are similar to the earlier theoretical results obtained using model effective potential based on projected valence bond method and using semi-empirical diatomics-in-molecules potential. The possible reason for discrepancies and the likely ways to improve the results are discussed in terms of the inclusion of higher excited electronic states into the dynamics calculation.

The Benzene−Argon Ground-State Intermolecular Potential Energy Surface Revisited

The benzene-Ar ground-state S(0) intermolecular potential energy surface is evaluated using the coupled cluster singles and doubles model including connected triple corrections and the augmented correlation consistent polarized valence triple-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. The surface is characterized by absolute minima of -390.1 cm(-1) where the argon atom is located on the benzene C(6) axis at distances of +/-3.536 A, and has a general shape close to the available ground-state S(0) and the first singlet S(1) and triplet T(1) excited-state surfaces. Using the potential, the intermolecular level structure of the complex is evaluated. The new intermolecular potential energy surface gives very accurate results and improves those previously available.

Suitability of Double Hybrid Density Functionals and Their Dispersion-Corrected Counterparts in Producing the Potential Energy Curves for CO2−Rg (Rg: He, Ne, Ar and Kr) Systems

The present work aims to establish the suitability of double hybrid density functionals in explaining the potential energy curves of carbon dioxide-rare gas (CO(2)-Rg; Rg: He, Ne, Ar, and Kr) systems. The interaction energies of the most stable T-shaped configuration of all CO(2)-Rg systems have been evaluated using pure gradient-corrected functionals and double hybrid density functionals and their dispersion-corrected analogs with the use of Dunning's augmented correlation consistent polarized valence triple-zeta (aug-cc-pVTZ) basis function. The equilibrium separation distance, r, between CO(2) and Rg obtained from the potential energy curves for these CO(2)-Rg systems are then compared with the experimental as well as with some earlier theoretical non-density functional theory (non-DFT) results. Our investigation suggests that for CO(2)-Ar/Kr systems, the r values obtained using the short-range corrected double hybrid mPW2PLYP functional is in excellent agreement with the experimental distances of separation. On the other hand, the short-range corrected double hybrid B2PLYP functional reproduces the experimental r values for the CO(2)-He/Ne systems quite satisfactorily. Interestingly, for lighter CO(2)-Rg (Rg: He and Ne) complexes, the B2PLYP functional fails to explain the potential energy surface, whereas the mPW2PLYP functional satisfactorily explains the potential well depth. On the other hand, for higher Rg complexes, none of the functionals are able to produce satisfactory potential well depth. Hence, the overall investigation suggests that, although double hybrid density functionals and other density functionals are good for predicting separation distance, they fail to produce correct interaction energy values in higher CO(2)-Rg complexes.