Ramón L. Panadés-Barrueta PhD defense
Simulations quantiques de l’interaction entre molécules atmosphériques et particules de suies modèle
Summary of the thesis:
We aim at simulating full quantum mechanically (nuclei and electrons) the processes of adsorption and photoreactivity of NO2 adsorbed on soot particles (modeled as large Polycyclic Aromatic Hydrocarbons, PAHs) in atmospheric conditions. A detailed description of these processes is necessary to understand the differential day-nighttime behavior of the production of HONO [1, 2], which is a precursor of the hydroxyl radical (OH) . In particular, the specific mechanism of the soot-mediated interconversion between NO2 and HONO is to date not fully understood. Due to its particular relevance in this context, we have chosen the Pyrene-NO2 system .
The first stage in this study has consisted in the determination of the stable configurations (transition states and minima) of the Pyrene-NO2 system. To this end, we have used the recently developed van der Waals Transition State Search using Chemical Dynamics Simulations (vdW-TSSCDS) method , the generalization of the TSSCDS algorithm [5, 6] developed in our group. In this way, the present work represents the first application of vdW-TSSCDS to a large system (81D). Starting from a set of judiciously chosen input geometries, the aforementioned method permits the characterization of the topography of an intermolecular Potential Energy Surface (PES), or in other words the determination of the most stable conformations of the system, in a fully automated and efficient manner.
The gathered topographical information has been used to obtain a global description (fit) of the interaction potential, necessary for the dynamical elucidation of the intermolecular interaction (physisorption), spectroscopic properties and reactivity of the adsorbed species. To achieve this last goal, we have developed two different methodologies together with the corresponding software packages. The first one of them is the Specific Reaction Parameter Multigrid POTFIT (SRP-MGPF) algorithm, which is implemented in the SRPTucker package . This method computes chemically accurate (intermolecular) PESs through reparametrization of semiempirical methods, which are subsequently tensor decomposed into Tucker form using MGPF . This software has been successfully interfaced with the Heidelberg version of the Multi-configuration Time-Dependent Hartree (MCTDH) package [9, 10]. The second method allows for obtaining the PES directly in the mathematical form required by MCTDH, thence its name Sum-Of-Products Finite-Basis-Representation (SOP-FBR) . SOP-FBR constitutes an alternative approach to NN-fitting methods. The idea behind it is simple: from the basis of a low-rank Tucker expansion on the grid, we replace the grid-based basis functions by an expansion in terms of a orthogonal polynomials. As in the previous method, an smooth integration with MCTDH has been ensured. Both methods have been successfully benchmarked with a number of reference problems, namely: the Hénon-Heiles Hamiltonian , a global H2O PES , and the HONO isomerization PES (6D) .
With the aid of all the above mentioned methods, we have tackled the computation of the global PES of the Pyrene-NO2 system. Suitable coordinate transformation routines have been developed in order to map the Cartesian coordinates to internal coordinates. In the physisorption domain, the evidence collected with vdW-TSSCDS have suggested that the geometry of the NO2 molecule is almost not perturbed in the stationary points with respect to the isolated molecule. This fact has enabled its treatment in a rigid monomer fashion (6D). The PESs will be used to obtain the electronic ground state (GS) and corresponding Zero-Point Energy (ZPE) of the system with MCTDH. The ZPE can offer an accurate estimate of the adsorption energy of the NO2 molecule over the Pyrene. Additionally, the electronic absorption spectrum of the system will be obtained by computing the sum (weighted by the GS distribution) of the individual vertical excitations of each stationary point.
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 Emilio Martínez-Núñez. An automated transition state search using classical trajectories initialized at multiple minima. Physical Chemistry Chemical Physics, 17 (22):14912–14921, 2015.
 Ramón Lorenzo Panadés-Barrueta, Emilio Martínez-Núñez, and Daniel Peláez. Specific reaction parameter multigrid potfit (SRP-MGPF): Automatic generation of sum-of-products form potential energy surfaces for quantum dynamical calculations. Frontiers in chemistry, 7:576, 2019.
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 Ramón Lorenzo Panadés-Barrueta and Daniel Peláez. Low-rank sum-of-products finite-basis-representation (SOP-FBR) of potential energy surfaces. The Journal of Chemical Physics, (in review), 2020.
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