Complex symmetrized calculations on ammonia vibrational levels

The full configuration interaction quantum Monte Carlo (FCIQMC) is a state-of-the-art stochastic electronic structure method, providing a methodology to compute FCI-level state energies of molecular systems within a quantum chemical basis. However, especially to probe dynamics at the FCIQMC level, it is necessary to devise more efficient schemes to produce nuclear forces and potential energy surfaces (PES) from FCIQMC. In this work, we derive the general formula for nuclear forces from FCIQMC, and clarify different contributions of the total force. This method to obtain FCIQMC forces eliminates previous restrictions and can be used with frozen core approximation and free selection of orbitals, making it promising for more efficient nuclear forces calculations. After some numerical checks of this procedure on the binding curve of N2 molecule, we use the FCIQMC energy and force to obtain the full-dimensional ground state PES of the water molecule via Gaussian processes regression. The new water FCIQMC PES can be used as the basis for H2O ground state nuclear dynamics, structure optimization, and rotation-vibrational spectrum calculation.

An alternative procedure for the calculation of highly excited vibrational levels in S0 formaldehyde was developed to apply to larger molecules. It is based on a new set of symmetrized vibrational valence coordinates. The fully symmetrized vibrational kinetic energy operator is derived in these coordinates using the Handy expression [Molec. Phys. 61, 207 (1987)]. The potential energy surface is expressed as a fully symmetrized quartic expansion in the coordinates. We have performed ab initio electronic computations using GAMESS to obtain all force constants of the S0 formaldehyde quartic force field. Our large scale vibrational calculations are based on a fully symmetrized vibrational basis set, in product form. The vibrational levels are calculated one by one using an artificial intelligence search/selection procedure and subsequent Lanczos iteration, providing access to extremely high vibrational energies. In this work special attention has been given to the CH stretch system by calculating the energies up to the fifth CH stretch overtone at ∼16000 cm−1, but the method has also been tested on two highly excited combination levels including other lower frequency modes.

The 30 lowest vibrational energy levels are calculated for the X̃ 3B1 and ã 1A1 states of CH2 and NH+2 for various potential energy surfaces. We include a new surface for the NH+2 triplet state, with predicted fundamentals ν1=3059, ν2=845, and ν3=3360 (expt 3359.9) cm−1. Where possible, results are compared with vibrational levels calculated using two other methods and with experiment. At low energy, the differences between the calculated vibrational levels are due primarily to the choice of potential energy surface. Of secondary importance is the method used to calculate the vibrational energy levels. The differences resulting from the method used are most apparent for cases with a diffuse vibrational wave function or for cases where the bond distance along the minimum energy path depends strongly on the bending angle.

A moving interpolation technique which provides an accurate representation of potential energy surfaces for polyatomic molecules is presented. The method uses the ab initio energy, energy gradient, and second derivatives calculated at dynamically important configurations. The interpolant of the energy and its derivatives converges to the exact value with increasing number of data. A procedure is given for finding the optimum configurations at which ab initio calculations are performed. The method is demonstrated by application to the six-dimensional surface of a diatomic plus diatomic reaction.

Towards efficient refinement of molecular potential energy surfaces: Ammonia as a case study

The first high-resolution spectra of ArHF excited to the vHF=2←0 manifold near 7800 cm−1 are recorded via direct infrared absorption in a slit supersonic expansion. The tunable difference frequency light is generated via nonlinear subtraction of a cw Nd:YAG laser from a tunable cw ring dye laser in temperature phase matched LiNbO3, and permits continuous single-mode access to the 1–2 μm near-IR region. Rotationally resolved spectra are presented for the pure HF stretching overtone (2000)←(0000), as well as for combination band excitation into the Σ bend (2100)←(0000) and Π bend (2110)←(0000) internal rotor levels built on the vHF=2 overtone stretch. Local perturbations in the Π bend spectrum are observed which arise from a resonant crossing of rotational levels with the (2002) van der Waals stretch and allow spectroscopic analysis of this state. Nonresonant coupling between the Σ and Π bend vibrational levels is evidenced by anomalous P branch/R branch transition intensities and is analyzed as Coriolis interactions in a tumbling, hindered rotor. The spectra reveal Doppler limited line shapes [Δν=79(11) MHz] characteristic of the temperature and geometry of the slit expansion. An upper limit of Δνprediss≤2 MHz Lorentzian broadening is established, indicating an 80 ns lower limit to the predissociation lifetime. Comparison of intermolecular vibrational levels in ArHF vHF=0, 1, and 2 indicates a systematic increase in both angular anisotropy and radial well depth upon excitation of the high-frequency HF stretch. In conjunction with previous results from the vHF=1 and vHF=0 vibrational levels, these studies provide the necessary data for fitting an atom+diatom potential energy surface as a function of all intermolecular and intramolecular internal degrees of freedom.

A previously reported potential energy surface (PES) and a new barrierless PES (both based on ab initio data and describing the CH3 group as a pseudoatom) were used to study the O(1D)+CH4→OH+CH3 reaction with the quasiclassical trajectory (QCT) method. The new PES accurately reproduces the experimental rate constant values, in contrast to the previous PES. The QCT study was mainly performed at the relative translational energy (ET) resulting from the photodissociation of N2O at 193 nm (〈ET〉=0.403 eV), although the collision energy obtained from the photodissociation of O3 at 248 nm (〈ET〉=0.212 eV) was also considered. Good agreement between theory and experiment was obtained for the OH vibrational populations and for the OH rotational populations for the v′⩾2 vibrational levels, while the rotational distributions for v′=0–1 are more excited than in the experiment. The QCT results at ET=0.403 eV satisfactorily reproduce the experimental kk′ angular distribution of the state-specific channel OH(v′=4, N′=8) and the corresponding ET′ distribution. For OH(v′=0, N′=5) the reproduction of these properties is poorer, especially for the ET′ distribution. At 0.403 eV the contribution of the abstraction mechanism to the reaction mode is negligible and two insertion like mechanisms (with fast or slow elimination) are found to be predominant, as suggested experimentally. The discrepancies observed between the QCT and experimental results can be explained on the basis of the defective description of the insertion/slow elimination mechanism provided by the model.

We obtain an accurate methane potential energy surface (PES) by starting with the ab initio PES of Schwenke and Partridge [Spectrochim. Acta A 57, 887 (2001)] and adjusting 5 of their parameters to reproduce 40 reliable experimentally determined vibrational levels of CH4. The 40 levels include all 35 levels in polyads up to and including the Octad and 5 levels in the Tetradecad. The Tetradecad levels are obtained from direct experimental transitions. The fit reduces the root mean square deviation of these 40 levels from 4.80 cm(-1) to 0.28 cm(-1). The new PES ought to aid in the analysis of the Tetradecad. To further test the accuracy of the new PES, vibrational levels are computed for CH4, CH3D, CHD3, and (13)CH4 and are compared with the extensive experimental data. The errors are all within about 1 cm(-1) except for a few cases.

K. C. Thompson and M. A. Collins, J. Chem. Soc., Faraday Trans., 1997, 93, 871 DOI: 10.1039/A606038B

TheoReTS – An information system for theoretical spectra based on variational predictions from molecular potential energy and dipole moment surfaces

Vibrational energy levels for CH 4 from an ab initio potential

The floating occupation molecular orbital-complete active space configuration interaction (FOMO-CASCI) method is a promising alternative to the state-averaged complete active space self-consistent field (SA-CASSCF) method. We have formulated the analytic first derivative of FOMO-CASCI in a manner that is well-suited for a highly efficient implementation using graphical processing units (GPUs). Using this implementation, we demonstrate that FOMO-CASCI gradients are of similar computational expense to configuration interaction singles (CIS) or time-dependent density functional theory (TDDFT). In contrast to CIS and TDDFT, FOMO-CASCI can describe multireference character of the electronic wavefunction. We show that FOMO-CASCI compares very favorably to SA-CASSCF in its ability to describe molecular geometries and potential energy surfaces around minimum energy conical intersections. Finally, we apply FOMO-CASCI to the excited state hydrogen transfer reaction in methyl salicylate.

Preface 1. Overview: Accurate Description of Low-Lying Electronic States and Potential Energy Surfaces 2. Simultaneous Account of Dynamic and Nondynamic Correlations Based on Complementarity of CI and CC Approaches 3. Method of Moments of Coupled-Cluster Equations: A New Theoretical Framework for Designing Black-Box Approaches for Molecular Potential Energy Surfaces 4. The Photoelectron Spectrum of the NO3 Radical Revisited: A Theoretical Investigation of Potential Energy Surfaces and Conical Intersections 5. Coupled Cluster Methods for Bond-Breaking 6. State-Specific Multireference Coupled Cluster-Based Methods for Potential Energy Surfaces and Their Approximate Variants 7. The Excited and Ion States of Allene 8. Hamiltonian Matrix Elements for the Table-CI Method Using Genealogical Configuration State Functions 9. Jacob's Ladder for Time-Dependent Density-Functional Theory: Some Rungs on the Way to Photochemical Heaven 10. Spectral Theory of Chemical Bonding 11. Comparison of CaF, ZnF, CaO, and ZnO: Their Anions and Cations in Their Ground and Low-Lying Excited States 12. The X1(2(3/2) and X2(2(1/2) Potentials of the Halogen Monoxides: A Comparison of RKR and Ab Initio Results 13. Symmetry in Spin-Orbit Coupling 14. Non-Adiabatic Bending Dissociation of OCS Induced by Orbital Unlocking 15. Electronic Structure and Dynamics in the O4 System 16. Model Studies of Intersystem Crossing Effects in the O + H2 Reaction 17. The Challenge of High-Resolution Dynamics: Rotationally Mediated Unimolecular Dissociation of HOCl 18. The Electronic Adiabatic-to-Diabatic Transformation Matrix and the Irreducible Representation of the Rotation Group 19. Spectroscopic Determination of Potential Energy Surfaces for the Out-of-Plane Ring Vibrations of Indan and Related Molecules in Their S0 and S1 ((, (*) States 20. Semiclassical Time Evolution in the Forward-Backward Stationary-Phase Limit 21. Development and Application of an Ab Initio Methane-Water Potential for the Study of Phase Equilibria of Methane Hydrates Indexes Author Index Subject Index

Theoretical study on the potential energy surface and vibrational energy levels of HXeI

Rotationally inelastic scattering of ND3 with Ar is studied at mean collision energies of 410 and 310 cm(–1). In the experimental component of the study, ND3 molecules are prepared by supersonic expansion and subsequent hexapole state selection in the ground electronic and vibrational levels and in the jk(±) = 1(1) rotational level. A beam of state-selected ND3 molecules is crossed with a beam of Ar, and scattered ND3 molecules are detected in single final j′k′(±) quantum states using resonance enhanced multiphoton ionization spectroscopy. State-to-state differential cross sections for rotational-level changing collisions are obtained by velocity map imaging. The experimental measurements are compared with close-coupling quantum-mechanical scattering calculations performed using an ab initio potential energy surface. The computed DCSs agree well with the experimental measurements, confirming the high quality of the potential energy surface. The angular distributions are dominated by forward scattering for all measured final rotational and vibrational inversion symmetry states. This outcome is in contrast to our recent results for inelastic scattering of ND3 with He, where we observed significant amount of sideways and backward scattering for some final rotational levels of ND3. The differences between He and Ar collision partners are explained by differences in the potential energy surfaces that govern the scattering dynamics.