Calculating Potential Energy Curves with Fixed-Node Diffusion Monte Carlo: CO and N₂

This study reports on the prospect for the routine use of Quantum Monte Carlo (QMC) for the electronic structure problem, applying fixed-node Diffusion Monte Carlo (DMC) to generate highly accurate Born-Oppenheimer potential energy curves (PECs) for small molecular systems. The singlet ground electronic states of CO and N2 were used as test cases. The PECs obtained by DMC employing multiconfigurational trial wavefunctions were compared with those obtained by conventional high-accuracy electronic structure methods such as multireference configuration interaction and/or the best available empirical spectroscopic curves. The goal was to test whether a straightforward procedure using available QMC codes could be applied robustly and reliably. Results obtained with DMC codes were found to be in close agreement with the benchmark PECs, and the n3 scaling with the number of electrons (compared with n7 or worse for conventional high-accuracy quantum chemistry) could be advantageous depending on the system size. Due to a large pre-factor in the scaling, for the small systems tested here, it is currently still much more computationally intensive to compute PECs with QMC. Nevertheless, QMC algorithms are particularly well-suited to large-scale parallelization and are therefore likely to become more relevant for future massively parallel hardware architectures

THE PREDICTION AND OBSERVATION OF VDW COMPLEXES OF HIGHLY VIBRATIONALLY EXCITED CS AND SIS WITH ARGON

Using a series of vibrationally effective PESs constructed using the automated AUTOSURF code,\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).} rovibrational levels and predicted microwave transition frequencies of the SiS--Ar and CS--Ar complexes were computed variationally. A series of shifting rotational transition frequencies were computed as a function of the diatom vibrational quantum number. For each system, the predicted spectra are compared with experimental microwave measurements

A New Set of Potential Energy Surfaces for HCO: Influence of Renner-Teller Coupling on the Bound and Resonance Vibrational States

It is commonly understood that the Renner-Teller effect can strongly influence the spectroscopy of molecules through coupling of electronic states. Here we investigate the vibrational bound states and low-lying resonances of the formyl radical treating the Renner-Teller coupled X2A\u27 and Ã2A states using the MultiConfiguration Time Dependent Hartree (MCTDH) method. The calculations were performed using the improved relaxation method for the bound states and a recently published extension to compute resonances. A new set of accurate global potential energy surfaces were computed at the explicitly correlated multireference configuration interaction (MRCI-F12) level and yielded remarkably close agreement with experiment in this application and thus enable future studies including photodissociation and collisional dynamics. The results show the necessity of including the large contribution from a Davidson correction in the electronic structure calculations in order to appreciate the relatively small effect of the Renner-Teller coupling on the states considered here

Calculated vibrational states of ozone up to dissociation

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.7]{ISMS2016.eps} \end{wrapfigure} A new accurate global potential energy surface for the ground electronic state of ozone [J. Chem. Phys. 139, 201103 (2013)] was published fairly recently. The topography near dissociation differs significantly from previous surfaces, without spurious submerged reefs and corresponding van der Waals wells. This has enabled significantly improved descriptions of scattering processes, capturing the negative temperature dependence and large kinetic isotope effects in exchange reaction rates. The exchange reactivity was found to depend on the character of near-threshold resonances and their overlap with reactant and product wavefunctions, which in turn are sensitive to the potential. Here we present global “three-well” calculations of all bound vibrational states of three isotopic combinations of ozone for J = 0 and J = 1 with a focus on the character and density of highly excited states. The calculations were done using a parallel symmetry-adapted Lanczos method with the RTR code, enabling the use of as many as 64.8 million basis functions. Tunneling splittings and the pseudorotation isomerization path will be discussed

State-To-State Inelastic Rotational Cross Sections in Five-Atom Systems with the Multiconfiguration Time Dependent Hartree Method

We present a MultiConfiguration Time Dependent Hartree (MCTDH) method as an attractive alternative approach to the usual quantum close-coupling method that approaches some computational limits in the calculation of rotational excitation (and de-excitation) between polyatomic molecules (here collisions between triatomic and diatomic rigid molecules). We have performed a computational investigation of the rotational (de-)excitation of the benchmark rigid rotor H2O-H2 system on a recently developed Potential Energy Surface of the complex using the MCTDH method. We focus here on excitations and de-excitations from the 000, 111, and 110 states of H2O with H2 in its ground rotational state, looking at all the potential transitions in the energy range 1-200 cm-1. This work follows a recently completed study on the H2O-H2 cluster where we characterized its spectroscopy and more generally serves a broader goal to describe inelastic collision processes of high dimensional systems using the MCTDH method. We find that the cross sections obtained from the MCTDH calculations are in excellent agreement with time independent calculations from previous studies but does become challenging for the lower kinetic energy range of the de-excitation process: that is, below approximately 20 cm-1 of collision energy, calculations with a relative modest basis become unreliable. The MCTDH method therefore appears to be a useful complement to standard approaches to study inelastic collision for various collision partners, even at low energy, though performing better for rotational excitation than for de-excitation

The Rotational Spectrum and Potential Energy Surface of the Ar-SiO Complex

The rotational spectra of five isotopic species of the Ar-SiO complex have been observed at high-spectral resolution between 8 and 18 GHz using chirped Fourier transform microwave spectroscopy and a discharge nozzle source; follow-up cavity measurements have extended these measurements to as high as 35 GHz. The spectrum of the normal species is dominated by an intense progression of a-type rotational transitions arising from increasing quanta in the Si-O stretch, in which lines up to v = 12 (~14 500 cm-1) were identified. A structural determination by isotopic substitution and a hyperfine analysis of the Ar-Si17O spectrum both suggest that the complex is a highly fluxional prolate symmetric rotor with a vibrationally averaged structure between T-shaped and collinear in which the oxygen atom lies closer to argon than the silicon atom, much like Ar-CO. To complement the experimental studies, a full dimensional potential and a series of effective vibrationally averaged, two-dimensional potential energy surfaces of Ar + SiO have been computed at the CCSD(T)-F12b/CBS level of theory. The equilibrium structure of Ar-SiO is predicted to be T-shaped with a well depth of 152 cm-1, but the linear geometry is also a minimum, and the potential energy surface has a long, flat channel between 140 and 180°. Because the barrier between the two wells is calculated to be small (of order 5 cm-1) and well below the zero-point energy, the vibrationally averaged wavefunction is delocalized over nearly 100° of angular freedom. For this reason, Ar-SiO should exhibit large amplitude zero-point motion, in which the vibrationally excited states can be viewed as resonances with long lifetimes. Calculations of the rovibrational level pattern agree to within 2% with the transition frequencies of normal and isotopic ground state Ar-SiO, and the putative Ka = ±1 levels for Ar-28SiO, suggesting that the present theoretical treatment well reproduces the salient properties of the intramolecular potential

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

A THEORETICAL CHARACTERIZATION OF ELECTRONIC STATES OF CH2IOO AND CH2OO RADICALS RELEVANT TO THE NEAR IR REGION

Criegee intermediates (R$_{1}$R$_{2}$COO or CIs) arise from ozonolysis of biogenic and anthropogenic alkenes, which is an important process in the atmosphere. Recent breakthroughs in producing them in the gas phase have resulted in a flurry of experimental and theoretical studies. Producing the simplest CI (CH$_{2}$OO) in the lab via photolysis of CH$_{2}$I$_{2}$ in the presence of O$_{2}$ yields both CH$_{2}$OO and CH$_{2}$IOO with pressure dependent branching. As discussed in the preceding talk, both species might be expected to have electronic transitions in the near IR (NIR). Here we discuss electronic structure calculations used to characterize the electronic states of both systems in the relevant energy range. Using explicitly-correlated multireference configuration interaction (MRCI-F12) and coupled-cluster (UCCSD(T)-F12b) calculations we were first able to exclude CH$_{2}$OO as the carrier of the observed NIR spectrum. Next, by computing frequencies and relaxed full torsional scans for the ~{A} and ~{X} states, we were able to aid in analysis and assignment of the NIR spectrum attributed to CH$_{2}$IOO

THE ROTATIONAL SPECTRUM AND POTENTIAL ENERGY SURFACE OF AR-SIO: A THEORETICAL INVESTIGATION

\begin{wrapfigure}{r}{0pt} \includegraphics[scale=0.55]{SiO_ISMS_caption.eps} \end{wrapfigure} The rotational spectra of five isotopic species of the Ar–SiO complex have been observed at high-spectral resolution, employing various techniques to obtain spectra between 8 and 35 GHz. Progressions of rotational transitions were recorded for a range of quanta in the Si-O stretch which correspond to resonance states of the complex since the vibrational frequency of the diatomic exceeds the binding energy of the complex. A complementary theoretical study was performed in which variational rovibrational calculations were performed using a series of potential energy surfaces (PESs) representing the SiO + Ar interaction and describing a range of vibrational quanta in the SiO(v=0,1…7) fragment. As seen in the Figure, the global minimum (V = -152.2 cm$^{-1}$) is nearly T-shaped, but a barrier of only 7.2 cm$^{-1}$ leads to a second minimum in a long channel along the angular coordinate. The relative energy of the T-shaped minimum and the channel toward linearity, varies with the number of quanta in SiO (progressively favoring the more linear structure), and for SiO(v=7), the T-shaped structure is no longer the global minimum. To compute the rovibrational levels and wavefunctions, the RV3 three-atom variational code of Wang and Carrington was used