THE AUTOMATED CONSTRUCTION OF POTENTIAL ENERGY SURFACES SUITABLE TO DESCRIBE VDW COMPLEXES OF HIGHLY EXCITED NASCENT REACTION PRODUCT MOLECULES

Some reactions producing extremely hot nascent products---with vibrational quantum numbers at least as high as 30---nevertheless form relatively long-lived weakly-bound van der Waals (vdW) complexes with bath gas molecules that are observable via microwave rotational spectroscopy. One example is SiO, formed in the reaction of various silanes with oxygen.\footnote{~M. C. McCarthy, S. A. Ndengu\'e and R. Dawes, The rotational spectrum and potential energy surface of the Ar--SiO complex, J. Chem. Phys. 149, 134308 (2018).} The reason for the long lifetimes of the complexes, despite having internal energies that greatly exceed the vdW well depth, is the very weak coupling between the intra- and intermolecular modes. Theoretical calculations of such unbound resonance states can be much more challenging than ordinary bound state calculations since approaches to deal with the dissociating wavefunction (such as complex absorbing potentials) are less straightforward and much more time consuming. 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 that are of spectroscopic accuracy. Here we will describe how our freely available PES fitting code called AUTOSURF\footnote{~E. Quintas-S\'anchez and R. Dawes, AUTOSURF: A Freely Available Program to Construct Potential Energy Surfaces, J. Chem. Inf. Model. 59, 262--271 (2019).}\footnote{~R. Dawes and E. Quintas-S\'anchez, The Construction of Ab Initio-Based Potential Energy Surfaces, Reviews in Computational Chemistry, Volume 31, Chapter 5, pp. 199-263, John Wiley \& Sons (2018).} can be used to construct the necessary PESs using automation. The code is demonstrated here by presenting spectroscopic-quality potential energy surfaces for Ar--CS and Ar--SiS complexes

AUTOSURF: A CODE FOR AUTOMATED CONSTRUCTION OF POTENTIAL ENERGY SURFACES

\begin{wrapfigure}{r}[0pt]{0pt} \includegraphics[scale=0.615]{corte2Dc1.eps} \end{wrapfigure} The potential energy surface (PES) of a molecular system constitutes a cornerstone for every theoretical study of spectroscopy and dynamics. We describe here our general code for the automated construction of PESs for van der Waals (vdW) systems composed of two (rigid) fragments. The AUTOSURF suite is designed to completely automate all of the steps and procedures that go into fitting various classes of PESs. The algorithms are based on the local interpolating moving least squares (L-IMLS) methodology, and have many advanced features such as options for data-point placement, and iterative refinement. We have interfaced this fitting approach to popular electronic structure codes such as Molpro and CFOUR to automatically generate ab initio PESs for 3D (atom - general molecule) and 4D (linear molecule - linear molecule) vdWs systems. The niche of these algorithms is to obtain an interpolative representation of high-level ab initio energies with negligible (arbitrarily small) fitting error, enabling a broad community of non-experts in PES fitting methods to bridge electronic structure calculations and spectroscopic and dynamics research. The code is demonstrated here by presenting PESs and analysis of the corresponding rovibrational bound states for 7 highly anisotropic “heavy-light” systems: C$_6$H$^-$-H$_2$, HC$_2$NC-H$_2$, HNC$_3$-H$_2$, HC$_5$N-H$_2$, C$_4$H$^-$-H$_2$, MgCCH-H$_2$, NCCP-H$_2$

A CODE FOR AUTOMATED CONSTRUCTION OF POTENTIAL ENERGY SURFACES FOR VAN DER WAALS SYSTEMS

begin{wrapfigure}{r}{0pt} _x000d_ includegraphics[scale=0.35]{fig2.eps} _x000d_ end{wrapfigure}_x000d_ _x000d_ The potential energy surface (PES) constitutes a cornerstone for theoretical studies of spectroscopy and dynamics. _x000d_ We fit PESs using a local interpolating moving least squares (L-IMLS) approach.footnote{M. Majumder, S. Ndengue and R. Dawes, Molecular Physics 114, 1 (2016).} _x000d_ The L-IMLS method is interpolative and has the flexibility to fit energies or energies and gradients, where inclusion of gradient information significantly reduces the number of points required for an accurate fit._x000d_ _x000d_ The method permits fully automated PES generation: _x000d_ beginning with an initial set of seed points, an automatic point selection scheme determines where new data are required and, in a series of iterations, computes new ab initio data and updates the fit until a specified accuracy is reached. _x000d_ We have interfaced this fitting approach to popular electronic structure codes such as Molpro and CFOUR to automatically generate ab initio 4D PESs for vdWs systems composed of two (rigid) linear fragments._x000d_ _x000d_ We present here our freely distributed code designed to run in parallel on a computing cluster, allowing the user to specify the system (masses, interatomic equilibrium distances, symmetry, energy range of interest, etc.) through an input file. _x000d_ For a selection of benchmark systems, we show that PESs with fitting errors below 1 wn, can be constructed using only a few hundred ab initio points

State-To-State Rate Coefficients for HCS\u3csup\u3e+\u3c/sup\u3ein Rotationally Inelastic Collisions with H2at Low Temperatures

HCS+ ions have been detected in several regions of the interstellar medium (ISM), but an accurate determination of the chemical-physical conditions in the molecular clouds where this molecule is observed requires detailed knowledge of the collisional rate coefficients with the most common colliders in those environments. In this work, we study the dynamics of rotationally inelastic collisions of HCS+ + H2 at low temperature, and report, for the first time, a set of rate coefficients for this system. We used a recently developed potential energy surface for the HCS+-H2 van der Waals complex and computed state-to-state rotational rate coefficients for the lower rotational states of HCS+ in collision with both para-and ortho-H2, analysing the influence of the computed rate coefficients on the determination of critical densities. Additionally, the computed rate coefficients are compared with those obtained by scaling the ones from HCS+ in collision with He (an approximation that is sometimes used when data is lacking), and large differences are found. Furthermore, the approximation of using the rates for the HCO+ + H2 collision as a rough approximation for those of the HCS+ + H2 system is also evaluated. Finally, the complete set of de-excitation rate coefficients for the lowest 30 rotational states of HCS+ by collision with H2 is reported from 5 to 100 K

Ab initio quantum scattering calculations and a new potential energy surface for the HCl(X1Σ+X^1\Sigma^+)-O2_{2}(X3Σg−X^3\Sigma^-_g) system: collision-induced line-shape parameters for O2_{2}-perturbed R(0) 0-0 line in H35^{35}Cl

The remote sensing of abundance and properties of HCl -- the main atmospheric reservoir of Cl atoms which directly participate in ozone depletion -- are important for monitoring the partitioning of chlorine between "ozone-depleting" and "reservoir" species. Such remote studies require knowledge of the shapes of molecular resonances of HCl, which are perturbed by collisions with the molecules of the surrounding air. In this work, we report the first fully quantum calculations of collisional perturbations of the shape of a pure rotational line in H$^{35}$Cl perturbed by an air-relevant molecule (as the first model system we choose the R(0) line in HCl perturbed by O$_2$). The calculations are performed on our new highly-accurate HCl($X^1\Sigma^+$)-O$_2$($X^3\Sigma^-_g$) potential energy surface. In addition to pressure broadening and shift, we determine also their speed dependencies and the complex Dicke parameter. This gives important input to the community discussion on the physical meaning of the complex Dicke parameter and its relevance for atmospheric spectra (previously, the complex Dicke parameter for such systems was mainly determined from phenomenological fits to experimental spectra and the physical meaning of its value in that context is questionable). We also calculate the temperature dependence of the line-shape parameters and obtain agreement with the available experimental data. We estimate the total combined uncertainties of our calculations at 2% relative RMSE residuals in the simulated line shape at 296~K. This result constitutes an important step towards computational population of spectroscopic databases with accurate ab initio line-shape parameters for molecular systems of terrestrial atmospheric importance.Comment: 15 pages, 7 figures, The following article has been accepted by The Journal of Chemical Physics. After it is published, it will be found at https://pubs.aip.org/aip/jc

Temperature Dependence Of The Electronic Absorption Spectrum Of NO2

The nitrogen dioxide (NO2) radical is composed of the two most abundant elements in the atmosphere, where it can be formed in a variety of ways including combustion, detonation of energetic materials, and lightning. Relevant also to smog and ozone cycles, together these processes span a wide range of temperatures. Remarkably, high-resolution NO2 electronic absorption spectra have only been reported in a narrow range below about 300 K. Previously, we reported [ J. Phys. Chem. A 2021, 125, 5519−5533 ] the construction of quasi-diabatic potential energy surfaces (PESs) for the lowest four electronic states (X̃, Ã, B̃, and C̃) of NO2. In addition to three-dimensional PESs based on explicitly correlated MRCI(Q)-F12/VTZ-F12 ab initio data, the geometry dependence of each component of the dipoles and transition dipoles was also mapped into fitted surfaces. The multiconfigurational time-dependent Hartree (MCTDH) method was then used to compute the 0 K electronic absorption spectrum (from the ground rovibrational initial state) employing those energy and transition dipole surfaces. Here, in an extension of that work, we report an investigation into the effects of elevated temperature on the spectrum, considering the effects of the population of rotationally and vibrationally excited initial states. The calculations are complemented by new experimental measurements. Spectral contributions from hundreds of rotational states up to N = 20 and from 200 individually-characterized vibrational states were computed. A spectral simulation tool was developed that enables modeling the spectrum at various temperatures─by weighting individual spectral contributions via the partition function, or for pure excited initial states, which can be probed via transient absorption spectroscopy. We validate these results against experimental absorption spectroscopy data at high temperatures, as well as via a new measurement from the (1,0,1) initial vibrational state

Surface Temperature Effects on the Dynamics of N₂ Eley-Rideal Recombination on W(100)

Quasiclassical trajectories simulations are performed to study the influence of surface temperature on the dynamics of a N atom colliding a N-preadsorbed W(100) surface under normal incidence. A generalized Langevin surface oscillator scheme is used to allow energy transfer between the nitrogen atoms and the surface. The influence of the surface temperature on the N2 formed molecules via Eley-Rideal recombination is analyzed at T = 300, 800, and 1500 K. Ro-vibrational distributions of the N2 molecules are only slightly affected by the presence of the thermal bath whereas kinetic energy is rather strongly decreased when going from a static surface model to a moving surface one. In terms of reactivity, the moving surface model leads to an increase of atomic trapping cross section yielding to an increase of the so-called hot atoms population and a decrease of the direct Eley-Rideal cross section. The energy exchange between the surface and the nitrogen atoms is semi-quantitatively interpreted by a simple binary collision model

Computational Study of the Ro-Vibrational Spectrum of CO-CO₂

An accurate ab initio ground-state intermolecular potential energy surface (PES) was determined for the CO-CO2 van der Waals dimer. The Lanczos algorithm was used to compute rovibrational energies on this PES. For both the C-in and O-in T-shaped isomers, the fundamental transition frequencies agree well with previous experimental results. We confirm that the in-plane states previously observed are geared states. In addition, we have computed and assigned many other vibrational states. The rotational constants we determine from J = 1 energy levels agree well with their experimental counterparts. Planar and out-of-plane cuts of some of the wavefunctions we compute are quite different, indicating strong coupling between the bend and torsional modes. Because the stable isomers are T-shaped, vibration along the out-of-plane coordinates is very floppy. In CO-CO2, when the molecule is out-of-plane, interconversion of the isomers is possible, but the barrier height is higher than the in-plane geared barrier height

Dynamical Reaction Pathways in Eley-Rideal Recombination of Nitrogen from W(100)

The scattering of atomic nitrogen over a N-pre-adsorbed W(100) surface is theoretically described in the case of normal incidence off a single adsorbate. Dynamical reaction mechanisms, in particular Eley-Rideal (ER) abstraction, are scrutinized in the 0.1-3.0 eV collision energy range and the influence of temperature on reactivity is considered between 300 and 1500 K. Dynamics simulations suggest that, though non-activated reaction pathways exist, the abstraction process exhibits a significant collision energy threshold (0.5 eV). Such a feature, which has not been reported so far in the literature, is the consequence of a repulsive interaction between the impinging and the pre-adsorbed nitrogens along with a strong attraction towards the tungsten atoms. Above threshold, the cross section for ER reaction is found one order of magnitude lower than the one for hot-atoms formation. The abstraction process involves the collision of the impinging atom with the surface prior to reaction but temperature effects, when modeled via a generalized Langevin oscillator model, do not affect significantly reactivity

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium

The Virtual Atomic and Molecular Data Centre (VAMDC) Consortium is a worldwide consortium which federates atomic and molecular databases through an e-science infrastructure and an organisation to support this activity. About 90% of the inter-connected databases handle data that are used for the interpretation of astronomical spectra and for modelling in many fields of astrophysics. Recently the VAMDC Consortium has connected databases from the radiation damage and the plasma communities, as well as promoting the publication of data from Indian institutes. This paper describes how the VAMDC Consortium is organised for the optimal distribution of atomic and molecular data for scientific research. It is noted that the VAMDC Consortium strongly advocates that authors of research papers using data cite the original experimental and theoretical papers as well as the relevant databases