Ab Initio-based Potential Energy Surface Research Articles

High-order contact transformations of molecular Hamiltonians: general approach, fast computational algorithm and convergence of ro-vibrational polyad models

The paper describes methods and fast computational algorithm for building effective Hamiltonians in molecular physics using perturbative approach. Separations of fast and slow variables are considered in the framework of contact transformations (CT). The particular focus is on a systematic derivation of effective models for rovibrational spectroscopy from ab initio-based potential energy surfaces with an exhaustive review of previous studies in this field. We consider applications to several types of polyads coupled by Fermi, Coriolis, Darling-Dennison and other types of resonance interactions with examples for asymmetric top, symmetric top and spherical top molecules. A flexible choice of the modelling operator accounts for strong couplings of various types of nuclear motion in molecules among closely lying levels including vibrational resonance schemes (2:1:2 . ), (2:1:2:1), (4:2:6:3), (3:2:1:2:1:1), etc. that occur for C2v, C3v and Td molecules and their isotopic species. The method is implemented in the MOL_CT programme suite, which offers a complementary tool to variational methods in terms of convergence and computational time. The range of applications is also different. The goal of the CT method is providing mathematical models for analyses of molecular spectra with the high-resolution accuracy using physically meaningful parameters derived from ab initio functions.

Ab Initio-Based Global Potential Energy Surface and Reaction Dynamics for H2S + C

Dynamics of the reaction of hydrogen sulfide, H2S(X1A1), with ground state atomic carbon, C(3Pj), was investigated over the interpolated ab initio based potential energy surface using the quasi-classical trajectory (QCT) simulation. The reaction probability and total reactive cross section were calculated at wide range of collision energies from 2.6 kJ mol-1 to 78.8 kJ mol-1. The total rate constant of the reaction was calculated using collision theory. The energy distribution for the formation of main products (HCS/HSC + H) were also investigated. At 44.6 kJ mol-1 collision energy approximately 39.5% and 12% of total available energy release to translational and rotational energy levels of the HCS + H products, respectively, while for HSC + H these values were found to be about 61.6% and 25.7% of total available energy. The rest of total energy deposited in the vibrational modes of the products.

Accurate CHIPR Potential Energy Surface for the Lowest Triplet State of C3.

We report the first global ab initio-based potential energy surface (PES) for ground-state triplet C3(3A') based on accurate energies extrapolated to the complete basis set (CBS) limit, and using the combined-hyperbolic-inverse-power-representation method for the analytical modeling. By relying on a cost-effective CBS(D,T) protocol, we ensure that the final form reproduces all topographical features of the PES, including its cyclic-linear isomerization barrier, with CBS(5,6)-quality. To partially account for the incompleteness of the N-electron basis and other minor effects, the available accurate experimental data on the relevant diatomics were used to obtain direct-fit curves that replace the theoretical ones in the many-body expansion. Besides describing properly long-range interactions at all asymptotic channels and permutational symmetry by built-in construction, the PES reported here reproduces the proper exothermicities at dissociation regions as well as the spectroscopy of the diatomic fragments. Bound vibrational state calculations in both linear and cyclic isomers have also been carried out, unveiling a good match of the available data on C3(ã 3Πu), while assisting with IR band positions for C3(3A2') that may serve as a guide for its laboratory and astronomical detection.

Quantum effects on the stability of the He5 I2 van der Waals conformers.

We present 15-dimensional quantum multiconfiguration time-dependent Hartree calculations of the vibrational levels of the He5 I2 van der Waals (vdW) complex employing an ab initio-based potential energy surface (PES). The energies and spatial features of such bound structures are analyzed, providing predictions on the structures and relative stabilities of its three lowest isomers. We found that the most stable isomer corresponds to all five He atoms encircling the I2 molecule, indicating that in this case the anharmonic quantum effects do not stabilize the isomers involving a He atom in a linear configuration as reported previously for the smaller HeN I2 systems. Such finding provides information on the overall structuring of the finite-size-solvent systems, highlighting the intriguing interplay between weak intermolecular interactions and quantum effects. © 2019 Wiley Periodicals, Inc.

Quasiclassical trajectory calculations of CD3CHO dissociation to CD2H + DCO on a global potential energy surface

We present a quasiclassical trajectory study of the photodissociation of CD3CHO based on a global ab initio-based potential energy surface. Calculations are performed at the total energy corresponding to the photolysis wavelength of 280[Formula: see text]nm. In addition to the major radical and molecular products, CD[Formula: see text] and CD3H [Formula: see text] CO, respectively, this paper focuses on the unusual radical channel CD2H [Formula: see text] DCO, which requires a D/H exchange process before the conventional C–C bond cleavage. Five D/H exchange mechanisms are reported, which are related to the isomerizations from acetaldehyde to vinyl alcohol and back, to oxirane and back, and to the intermediate (CD–CHD–OD) and back. These D/H exchange mechanisms are in good agreement with the experimental findings [Heazlewood BR, Maccarone AT, Andrews DU, Osborn DL, Harding LB, Klippenstein SJ, Jordan MJT, Kable SH, Nat Chem 3:443, 2011].

Accurate ab initio-based double many-body expansion potential energy surface for the adiabatic ground-state of the C3 radical including combined Jahn-Teller plus pseudo-Jahn-Teller interactions.

A fully ab initio-based potential energy surface is first reported for the ground electronic state of the C3 radical using the double many-body expansion (DMBE) method. The DMBE form so obtained mimics the full set of energies calculated at the multireference configuration interaction level of theory with chemical accuracy. To account for the incompleteness of the one- and N-electron bases, the calculated external correlation energies have been scaled prior to the fitting procedure via DMBE-scaled external correlation method. Furthermore, the novel potential energy surface reproduces accurately dissociation energies, diatomic potentials, long-range interactions at all asymptotic channels, and the correct topological behavior at the region of 4 conical intersections with the partner state of the same symmetry near equilateral triangular geometries due to combined Jahn-Teller (E' ⊗ e') plus pseudo-Jahn-Teller [(E'+A1')⊗e'] interactions. Rovibrational calculations have also been performed, unveiling a good match of the vibrational spectrum of C3 for 53 calculated levels. The present DMBE form is, therefore, commended for both spectroscopic and reaction dynamics studies, some also performed in the present work.

Thermal Rate Constants for the O(3P) + HBr and O(3P) + DBr Reactions: Transition-State Theory and Quantum Mechanical Calculations

The O((3)P) + HBr → OH + Br and O((3)P) + DBr → OD + Br reactions are studied on a recent high-quality ab initio-based potential energy surface. Thermal rate constants over the 200-1000 K temperature range, calculated using variational transition-state theory (VTST) with the small-curvature tunneling (SCT) correction and quantum mechanical methods with the J-shifting approximation (QM/JS) for zero total angular momentum (J = 0), are reported. These results are compared to the available experimental data, which lie in the ranges of 221-554 and 295-419 K for O + HBr and O + DBr, respectively. The rate constants, in cm(3) molecule(-1) s(-1) and at 298 K, for the O + HBr reaction are 3.66 × 10(-14) for VTST, 3.80 × 10(-14) for QM/JS, and 3.66 × 10(-14) for the average of eight experimental measurements.

Quasiclassical trajectory study of fast H-atom collisions with acetylene

Translationally hot H collisions with the acetylene are investigated using quasiclassical trajectory calculations, on a recent full-dimensional ab initio-based potential energy surface. Three outcomes are focused on: non-reactive energy transfer via prompt collisions, non-reactive energy transfer via the formation of the vinyl complex, and reactive chemical H-atom exchange, also via complex formation. The details of these outcomes are presented and correlated with the collision lifetime. Large energy transfer is found via complex formation, which can subsequently decay back to reactants, a non-reactive event, or to new products, a reactive event. For the present system, these two events are experimentally indistinguishable.

Quasiclassical Trajectory Calculations of the Dissociation Dynamics of CH3CHO at High Energy Yield Many Products

We present a quasiclassical trajectory study of the photodissociation of CH3CHO using a global ab initio-based potential energy surface. Calculations are performed at a total energy of 160 kcal/mol...

Ro-vibrational spectra of C2H2 based on variational nuclear motion calculations

A published ab initio-based potential energy surface and newly constructed dipole moment surface of acetylene have been used to compute vibrational band intensities. The line intensity calculations employed the variational nuclear motion code WAVR4 for computation of wave functions and energy levels, and a newly developed code DIPOLE4 for computation of dipole transitions. Owing to the high computational cost of J > 0 transitions using direct variational methods only J = 0 and J = 1 states and transitions have been computed variationally. The intensities of J > 1 transitions were extrapolated from J = 0 and J = 1 using Hönl–London coefficients. The resulting effective rotational constants B and transition intensities are compared with experimental data for the (3ν4 + ν5) combination band, the ν3 and the ν5 fundamental band. The prospects of using this procedure for extensive calculations of a hot line list, important for cool stars and extrasolar planets are discussed.

Communications: Classical trajectory study of the postquenching dynamics of OH A ∑2+ by H2 initiated at conical intersections

We report results of a classical trajectory calculation of the postquenching dynamics of OH A (2)Sigma(+) by H(2). This is done by performing roughly 100 000 trajectories at previously identified conical intersections (CoIs) [B. C. Hoffman and D. R. Yarkony, J. Chem. Phys. 113, 10091 (2000)]. The initial momenta are sampled fully and partially microcanonically, corresponding to "adiabatic" and "diabatic" model of the dynamics, respectively. The trajectories are propagated on a new ground state ab initio-based potential energy surface. This surface is a permutationally invariant fit to roughly 23,000 electronic energies (multireference configuration interaction/correlation-consistent-augmented-triple-zeta basis) at configurations obtained mostly from direct-dynamics calculations (complete active space second order perturbation theory with correlation-consistent-augment double-zeta basis), also initiated at the CoIs. Final rovibrational state distributions of the ground electronic state OH product and the H-atom translational energy distributions for abstraction and insertion mechanisms are calculated and compared to experimental ones. Agreement for these observable quantities is good. The branching between reactive and nonreactive quenching is sensitive to the momenta sampling; very good agreement with experiment is obtained using the diabatic sampling but not with the adiabatic sampling. The calculated H(2) rovibrational distributions (not measured experimentally) are also presented.

Full-dimensional ab initio potential energy surface and vibrational configuration interaction calculations for vinyl

The potential energy landscape and two permutationally invariant, full-dimensional ab initio-based potential energy surfaces (PESs) for the doublet vinyl radical, C(2)H(3), are described. The first of the two surfaces, denoted as PES/S, describes the equivalent CH(2)CH global minimum and the saddle point separating them, planar and nonplanar H-atom migration saddle points, a methylcarbyne local minimum that is due to a Jahn-Teller conical intersection, and the saddle point connecting it with the global minimum. The second PES, denoted PES/D, contains all stationary points of PES/S and in addition describes dissociation to C(2)H(2)+H fragments, including the saddle point to dissociation along a least-energy path. The surfaces are least-squares fits to electronic energies obtained with use of the spin-restricted coupled cluster singles and doubles with perturbative treatment of triples method and augmented correlation consistent polarized valence triple zeta basis sets, using permutationally invariant polynomials in "Morse variables" and a many-body expansion. PES/S is a fit to roughly 34,000 and PES/D to roughly 50,000 electronic structure energies. PES/S is used in full-dimensional, vibrational configuration interaction calculations of the vinyl zero-point energy and fundamental vibrational energies, which are compared to recent experiments.

New insights on reaction dynamics from formaldehyde photodissociation

We review the photodissociation dynamics of formaldehyde with an emphasis on recent calculations that make use of a global ab initio-based potential energy surface for the S(0) state. These calculations together with recent experiments reveal striking departures from conventional transition state theory for the formation of the molecular products H(2) + CO. The evidence for this departure is reviewed in detail by examining properties of the new potential surface and results of quasiclassical trajectory dynamics calculations using this surface. We also review very recent work on the dynamics governing the formation of radical products, H + HCO. These products can be formed on the T(1) surface as well as the S(0) one, and we present some results contrasting the dynamics on these two surfaces. This work makes use of a new semi-global ab initio-based T(1) potential energy surface.

Quasiclassical trajectory studies of the dynamics of H2CO on a global ab initio-based potential energy surface

This paper reports the results of quasiclassical molecular dynamics studies of H2CO → H2+CO, as well as the corresponding isotopomers, on a new potential energy surface. Although the surface describes the H+HCO channel, the energy for the unimolecular reaction is restricted to be below the threshold for this channel. This is done so that direct comparisons can be made with corresponding experiments that have determined the final rotational and vibrational states for the CO and H2 products. To demonstrate the global quality of the potential, calculations are presented for zero-impact parameter collisions for H+HCO → H2+CO at one relative collision energy, although no experiments have been reported for this reaction.