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1. Reduced-dimensional surface hopping with offline–online computations

   https://doi.org/10.1039/D1CP03446D  

   Morrow, Zachary; Kwon, Hyuk-Yong; Kelley, C. T.; Jakubikova, Elena (August 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Molecular dynamics simulations often classically evolve the nuclear geometry on adiabatic potential energy surfaces (PESs), punctuated by random hops between energy levels in regions of strong coupling, in an algorithm known as surface hopping. However, the computational expense of integrating the geometry on a full-dimensional PES and computing the required couplings can quickly become prohibitive as the number of atoms increases. In this work, we describe a method for surface hopping that uses only important reaction coordinates, performs all expensive evaluations of the true PESs and couplings only once before simulating dynamics (offline), and then queries the stored values during the surface hopping simulation (online). Our Python codes are freely available on GitHub. Using photodissociation of azomethane as a test case, this method is able to reproduce experimental results that have thus far eluded ab initio surface hopping studies. 

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2. Automated Construction of Potential Energy Surfaces Suitable to Describe van der Waals Complexes With Highly-Excited Nascent Molecules: The Rotational Spectra of Ar–CS( v ) and Ar–SiS( v )

   https://doi.org/10.1021/acs.jpca.0c02685  

   Quintas-Sánchez, Ernesto; Dawes, Richard; Lee, Kelvin; McCarthy, Michael C (May 2020, The Journal of Physical Chemistry A)

   Some reactions produce extremely hot nascent-products which nevertheless can form sufficiently long-lived van der Waals (vdW) complexes—with atoms or molecules from a bath gas—as to be observed via microwave spectroscopy. Theoretical calculations of such unbound resonance-states can be much more challenging than ordinary bound-state calculations depending on the approach employed. One encounters not only the floppy, and perhaps multi-welled potential energy surface (PES) characteristic of vdWs complexes, but in addition must contend with excitation of the intramolecular modes and its corresponding influence on the PES. Straightforward computation of the (resonance) rovibrational levels of interest, involves the added complication of the unbound nature of the wavefunction, often treated with techniques such as introducing a complex absorbing potential. Here, we have demonstrated that a simplified approach of making a series of vibrationally effective PESs for the intermolecular coordinates—one for each reaction product vibrational quantum number of interest—can produce vdW levels for the complex with spectroscopic accuracy. This requires constructing a series of appropriately weighted lower-dimensional PESs for which we use our freely available PES-fitting code AUTOSURF. The applications of this study are the Ar–CS and Ar–SiS complexes, which are isovalent to Ar–CO and Ar–SiO, the latter of which we considered in a previously reported study. Using a series of vibrationally effective PESs, rovibrational levels and predicted microwave transition frequencies for both complexes were computed variationally. A series of shifting rotational transition frequencies were also computed as a function of the diatom vibrational quantum number. The predicted transitions were used to guide and inform an experimental effort to make complementary observations. Comparisons are given for the transitions that are within the range of the spectrometer and were successfully recorded. Calculations of the rovibrational level pattern agree to within 0.2 % with experimental measurements. 

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3. Incorporating reduced axis system embedding into ab initio tunnelling-rotation Hamiltonians with curvilinear vibrational Møller–Plesset perturbation theory

   https://doi.org/10.1080/00268976.2024.2304098  

   Changala, P Bryan (January 2024, Molecular Physics)

   Large-amplitude vibrational motion influences the rovibrational structure of molecules that tunnel between multiple wells. Reaction path (RP) Hamiltonians, and curvilinear coordinates more gen- erally, are useful for modelling pure vibrational motion in these systems and provide a practical framework for calculating accurate ab initio anharmonic vibrational energies and tunnelling split- tings with perturbation theory. These computational tools also offer the means to address rotation- vibration coupling associated with large-amplitude motion in rotating molecules. In this paper, we incorporate the reduced axis system (RAS) frameembeddingwithRPHamiltoniansandsecond-order vibrational Møller-Plesset perturbation theory (VMP2). Because the RP-RAS Hamiltonian eliminates rotation-vibration momentumcoupling everywhere along a one-dimensional reaction path, it is well suited for rovibrational VMP2 methods, the convergence of which relies critically on approximate vibration-vibration and vibration-rotation separability. The accuracy of this combined RP-RAS-VMP2 scheme is demonstrated by comparisons with numerically exact variational calculations and VMP2 parameters based on traditional Eckart embeddings for reduced-dimension models of torsional tunnelling in hydrogenperoxideandinversion tunnelling in cyclopropyl radical. Thefavourablecom- putational scaling ofVMP2makes it a promising strategy for calculating accurate tunnelling-rotation parameters for medium-sized and larger molecules in full dimensionality. 

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4. Intermolecular vibrational states of HF trimer from rigorous nine-dimensional quantum calculations: Strong coupling between intermolecular bending and stretching vibrations and the importance of the three-body interactions

   https://doi.org/10.1063/5.0128550  

   Felker, Peter M.; Bačić, Zlatko (November 2022, The Journal of Chemical Physics)

   We present the computational methodology that allows rigorous and efficient nine-dimensional (9D) quantum calculations of the intermolecular vibrational states of noncovalently bound trimers of diatomic molecules, with the monomers treated as rigid. The full 9D vibrational Hamiltonian of the trimer is partitioned into a 3D “frame” (or stretching) Hamiltonian and a 6D “bend” Hamiltonian. These two Hamiltonians are diagonalized separately, and a certain number of their lowest-energy eigenstates is included in the final 9D product contracted basis in which the full 9D intermolecular vibrational Hamiltonian is diagonalized. This methodology is applied to the 9D calculations of the intermolecular vibrational levels of (HF)3, a prototypical hydrogen-bonded trimer, on the rigid-monomer version of an ab initio calculated potential energy surface (PES). They are the first to include fully the stretch-bend coupling present in the trimer. The frequencies of all bending fundamentals considered from the present 9D calculations are about 10% lower than those from the earlier quantum 6D calculations that considered only the bending modes of the HF trimer. This means that the stretch-bend coupling is strong, and it is imperative to include it in any accurate treatment of the (HF)3 vibrations aiming to assess the accuracy of the PES employed. Moreover, the 9D results are in better agreement with the limited available spectroscopic data that those from the 6D calculations. In addition, the 9D results show sensitivity to the value of the HF bond length, equilibrium or vibrationally averaged, used in the calculations. The implication is that full-dimensional 12D quantum calculations will be required to obtain definitive vibrational excitation energies for a given PES. Our study also demonstrates that the nonadditive three-body interactions are very significant in (HF)3 and have to be included in order to obtain accurate intermolecular vibrational energy levels of the trimer. 

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5. The Li + CaF → Ca + LiF chemical reaction under cold conditions

   https://doi.org/10.1039/d3cp01464a  

   da Silva, Humberto; Yao, Qian; Morita, Masato; Kendrick, Brian K.; Guo, Hua; Balakrishnan, Naduvalath (May 2023, Physical Chemistry Chemical Physics)

   The calcium monofluoride (CaF) molecule has emerged as a promising candidate for precision measurements, quantum simulation, and ultracold chemistry experiments. Inelastic and reactive collisions of laser cooled CaF molecules in optical tweezers have recently been reported and collisions of cold Li atoms with CaF are of current experimental interest. In this paper, we report ab initio electronic structure and full-dimensional quantum dynamical calculations of the Li + CaF → LiF + Ca chemical reaction. The electronic structure calculations are performed using the internally contracted multi-reference configuration-interaction method with Davidson correction (MRCI + Q). An analytic fit of the interaction energies is obtained using a many-body expansion method. A coupled-channel quantum reactive scattering approach implemented in hyperspherical coordinates is adopted for the scattering calculations under cold conditions. Results show that the Li + CaF reaction populates several low-lying vibrational levels and many rotational levels of the product LiF molecule and that the reaction is inefficient in the 1–100 mK regime allowing sympathetic cooling of CaF by collisions with cold Li atoms. 

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