Vibrational energy flow in molecules, like the dynamics of other many dimensional finite systems, involves quantum transport across a dense network of near resonant states. For molecules in their electronic ground state, the network is ordinarily provided by anharmonic vibrational Fermi resonances. Surface crossing between different electronic states provides another route to chaotic motion and energy redistribution. We show that nonadiabatic coupling between electronic energy surfaces facilitates vibrational energy flow, and conversely, anharmonic vibrational couplings facilitate nonadiabatic electronic state mixing. A generalization of the Logan-Wolynes theory of quantum energy flow in many-dimensional Fermi resonance systems to the two-surface case gives a phase diagram describing the boundary between localized quantum dynamics and global energy flow. We explore these predictions and test them using a model inspired by the problem of electronic excitation energy transfer in the photosynthetic reaction center. Using an explicit numerical solution of the time dependent Schrödinger equation for this ten-dimensional model, we find quite good agreement with the expectations from the approximate analytical theory.

We have developed a black-box method to evaluate vibrational spectra using only the information provided by semi-classical ab initio molecular dynamics simulations. Using a distributed Gaussian basis set centered at the points sampled by an AIMD trajectories and a local harmonic approximation to the potential at each point, it is possible to obtain accurate vibrational spectra. By exploiting the locality of molecular vibrations, this method is applicable to the evaluation of spectra systems of moderate size. The judicious choice of Gaussian width parameters as well as selection augmentation of the basis with appropriate harmonic basis functions can lead to high-fidelity spectra even for relatively short trajectories. This method provides a complement to perturbative approaches, as it treats low-frequency vibrations accurately and is amenable to systems with multiple low-lying energetic minima. Furthermore, by running more AIMD trajectories, it is possible to refine the vibrational spectrum obtained.

I will review our recent (up to one or two years) and very recent (months and days) results on the line intensity calculations and comparison with high accuracy and extra high accuracy experimental observations. High accuracy is about 1 % and extra high accuracy is better than 0.1 % (pro-mille). I will focus mostly on CO and CO₂ line intensities. Time permitting the results on water, ozone, HCN and N₂O will be mentioned.

The fields of electronic and vibrational structure theory have largely evolved independently over the years. Demonstrative of this is the fact that the electronic structure community has generally embraced coupled-cluster wavefunctions as the gold standard for small, well-behaved systems, while vibrational structure practitioners tend to prefer treatments based on vibrational perturbation theory or vibrational configuration interaction. While this is not surprising — the two fields face dramatically different challenges and goals — one does wonder if ideas from one can be used to improve or inform the other. To this point, Part I of this presentation borrows the concept of normal-ordered strings of creation/annihilation operators from electronic coupled-cluster to develop a new form of the vibrational Hamiltonian which is amenable to vibrational many-body (VMP,VCI,VCC) calculations. This "GEN" Hamiltonian generates greatly simplified equations to be implemented in black-box software, and folds-in higher-order many-body effects into lower-order treatments. The goal is to present these (somewhat complicated) concepts in a way accessible to theorists from both communities, as well as to the broader field of spectroscopists as a whole.

The fields of electronic and vibrational structure theory have largely evolved independently over the years. Demonstrative of this is the fact that the electronic-structure community has generally embraced coupled-cluster wavefunctions as the gold standard for small, well-behaved systems, while vibrational structure practitioners tend to prefer treatments based on vibrational perturbation theory or configuration interactions. While this is not surprising — the two fields face dramatically difference challenges and goals — one does wonder if ideas from one can be used to improve or inform the other. To this point, Part II of this presentation borrows the diagrammatic techniques of electronic coupled-cluster to develop a set of Davidson-like corrections to the vibrational CI wavefunction, which help resolve some of the size-consistency problems that can arise in VCI calculations. The goal is to present these (somewhat complicated) concepts in a way accessible to theorists from both fields, as well as to the broader field of spectroscopists as a whole.

The availability of integrated solutions in user-friendly computational packages coupled with capabilities of modern hardware have facilitated the inclusion of anharmonicity in the simulation of vibrational spectra over the recent years. Cost-effective methods such as the second-order vibrational perturbation theory (VPT2) can even be applied to molecular systems comprising dozens of atoms. However, the well-documented problem of resonances, their identification and correction remain a critical pitfall of perturbative methods. Recent works have highlighted the sensitivity of band intensities to even subtle resonance effects, underlying the importance of a correct treatment to predict accurate spectral band-shapes.[1] This aspect is even more critical with chiroptical spectroscopies whose signal is weak. In this contribution, we analyze the impact of resonances and explore strategies to identify and correct them, not only in energy calculations, but also on the transition moments.[2] A selection of representative molecules of different sizes was used for the study. We show how resonances can affect the overall spectral band-shapes, especially on chiroptical spectroscopies, and the accuracy achievable once they are properly treated, even beyond the fingerprint region.[3,4]

The vinoxy radical is an important intermediate in the combustion of hydrocarbon fuels, however, experimental investigations have faced considerable challenges in obtaining estimates of desired thermochemical quantities to within 10 kJ/mol. Computational studies thus account for the bulk of the Active Thermochemical Tables (ATcT) provenance for vinoxy, with the uncertainty of the ATcT enthalpy of formation lingering near 0.6 kJ/mol. In an attempt to reduce the uncertainty of these quantities, we apply an extended version of the HEAT model chemistry currently under development to the vinoxy radical and its associated cations. These treatments elucidate bond energies of small molecules containing first- and second-row atoms to within 20 cm⁻¹. Composite techniques provide very accurate zero-point energies for use in the thermochemical protocol and fundamental vibrational frequencies that are in excellent agreement with experiment. Anharmonic resonances are reanalyzed, suggesting an uncharacteristically complex CH stretching region. To compound matters, very little information is available concerning the photoionization spectrum of vinoxy, which is thought to undergo a large geometry change upon ionization. We report the adiabatic ionization energy for the vinoxy radical and a simulated photoionization spectrum generated from a harmonic autocorrelation function approach.

We present spinor-based relativistic equation-of-motion coupled-cluster (EOM-CC) calculations for the actinide M-edge x-ray absorption spectra for uranyl, neptunyl, and plutonyl-containing molecules. An efficient implementation of core-valence separation for the spinor-based EOM-CC methods have enabled the calculations of the x-ray absorption spectra with rigorous treatments of relativistic and correlation effects. The benchmark calculations demonstrate the importance of the spin-orbit coupling and solvation effects on the computed spectra. The computed spectra are then compared with high-resolution x-ray absorption spectra extracted from the resonant inelastic x-ray spectroscopy (RIXS) map. The covalency of actinide 5f electrons is discussed.

The AI ENERGIES database was first made available on GitHub in January 2018, and by February 2023 has had more than 600 commits involving ab initio ground and excited state energies for various atoms and molecules with various charges, spin multiplicities and geometries. It attempts to make available all ab initio calculations ever done by participants, and to use artificial intelligence to predict energies for systems that are too difficult for running full ab initio calculations. What began as a repository to preserve output files and summaries of various calculated energies for the Li₂ molecule, and to make them available for artificial intelligence training, has grown now to include molecules as big as C₁₂H₁₀B₂N₂, and state-of-the-art benchmark calculations on systems as small as the H atom but with basis sets as large as aug-cc-pV10Z. The repository also contains an FCI database which attempts to curate and make available all full configuration interaction energies known, and it contains a coupled cluster (CC) database which attempts to include an unusually thorough level of detail about all coupled cluster calculations ever done by participants. By including the maximum RAM and CPU time used, along with as many details about the system for which an FCI or CC calculation is performed, and details about the hardware used, estimations can be made in advance for the cost (or feasibility) of doing for example a CCSDTQ(P) calculation on a singlet C_2v system with 8 correlated electrons in 200 spatial orbitals with three CPU threads: the estimation in this case would be approximately two weeks with 147GB of RAM. In this way, scientists can refer to the database and the predictions that it is able to provide, which can help with the planning of projects and the evaluation of a project's feasibility. The repository additionally contains a GENBAS file which contains 70,000+ lines of Gaussian basis set exponents and contraction coefficients with an unusually large amount of care towards reproducibility and inclusion of some of the most coveted and specialized basis sets that are not available in the ordinary basis set repositories available online. Anyone is able to contribute data to the AI ENERGIES database on GitHub (https://github.com/HPQC-LABS/AI_ENERGIES), and this can help them preserve and organize data that might otherwise get accidentally deleted or lost. The database also has a Digital Object Identifier (DOI): https://doi.org/10.5281/zenodo.5529103

The super-correlation consistent composite approach (s-ccCA) has been successful in the prediction of accurate dissociation energies for early 3d and 4d transition metal-containing species Mol. Phys. 119, (2021) As one moves across the periodic table the computational cost requirements also increase. Thus, this increase in computational cost is an important consideration in the development of composite methodologies. Here we present s-ccCA for the late 3d and 4d transition metal molecules with an important modification, employing a simple Continued Fraction approximant. This approach was evaluated against s-ccCA results on the early 3d and 4d species. This reduced cost s-ccCA was then applied to the late 3d and 4d species, and the resulting dissociation energies were compared with state-of-the-art Resonant Two-Photon Ionization bond dissociation energies J. Chem. Phys. 153, 074303, (2020)J. Chem. Phys. 146, 144310, (2017)J. Chem. Phys. 151, 044302, (2019) Mol. Phys. 119, (2021). J. Chem. Phys. 153, 074303, (2020)

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J. Chem. Phys. 151, 044302, (2019).