Ab initio calculation of H + He+^+ charge transfer cross sections for plasma physics

The charge transfer in low energy (0.25 to 150 eV/amu) H($nl$) + He$^+(1s)$ collisions is investigated using a quasi-molecular approach for the $n=2,3$ as well as the first two $n=4$ singlet states. The diabatic potential energy curves of the HeH$^+$ molecular ion are obtained from the adiabatic potential energy curves and the non-adiabatic radial coupling matrix elements using a two-by-two diabatization method, and a time-dependent wave-packet approach is used to calculate the state-to-state cross sections. We find a strong dependence of the charge transfer cross section in the principal and orbital quantum numbers $n$ and $l$ of the initial or final state. We estimate the effect of the non-adiabatic rotational couplings, which is found to be important even at energies below 1 eV/amu. However, the effect is small on the total cross sections at energies below 10 eV/amu. We observe that to calculate charge transfer cross sections in a $n$ manifold, it is only necessary to include states with $n^{\prime}\leq n$, and we discuss the limitations of our approach as the number of states increases.Comment: 14 pages, 10 figure

H2, HD, and D2 in the small cage of structure II clathrate hydrate: vibrational frequency shifts from fully coupled quantum six-dimensional calculations of the vibration-translation-rotation eigenstates

We report the first fully coupled quantum six-dimensional (6D) bound-state calculations of the vibration-translation-rotation eigenstates of a flexible H2, HD, and D2 molecule confined inside the small cage of the structure II clathrate hydrate embedded in larger hydrate domains with up to 76 H2O molecules, treated as rigid. Our calculations use a pairwise-additive 6D intermolecular potential energy surface for H2 in the hydrate domain, based on an ab initio 6D H2–H2O pair potential for flexible H2 and rigid H2O. They extend to the first excited (v = 1) vibrational state of H2, along with two isotopologues, providing a direct computation of vibrational frequency shifts. We show that obtaining a converged v = 1 vibrational state of the caged molecule does not require converging the very large number of intermolecular translation-rotation states belonging to the v = 0 manifold up to the energy of the intramolecular stretch fundamental (≈4100 cm−1 for H2). Only a relatively modest-size basis for the intermolecular degrees of freedom is needed to accurately describe the vibrational averaging over the delocalized wave function of the quantum ground state of the system. For the caged H2, our computed fundamental translational excitations, rotational j = 0 → 1 transitions, and frequency shifts of the stretch fundamental are in excellent agreement with recent quantum 5D (rigid H2) results [A. Powers et al., J. Chem. Phys. 148, 144304 (2018)]. Our computed frequency shift of −43 cm−1 for H2 is only 14% away from the experimental value at 20 K

Ab initio calculation of the 66 low lying electronic states of HeH+^+: adiabatic and diabatic representations

We present an ab initio study of the HeH$^+$ molecule. Using the quantum chemistry package MOLPRO and a large adapted basis set, we have calculated the adiabatic potential energy curves of the first 20 $^1 \Sigma^+$, 19 $^3\Sigma^+$, 12 $^1\Pi$, 9 $^3\Pi$, 4 $^1\Delta$ and 2 $^3\Delta$ electronic states of the ion in CASSCF and CI approaches. The results are compared with previous works. The radial and rotational non-adiabatic coupling matrix elements as well as the dipole moments are also calculated. The asymptotic behaviour of the potential energy curves and of the various couplings between the states is also studied. Using the radial couplings, the diabatic representation is defined and we present an example of our diabatization procedure on the $^1\Sigma^+$ states.Comment: v2. Minor text changes. 28 pages, 18 figures. accepted in J. Phys.

Explicitly correlated treatment of H2NSi and H2SiN radicals: Electronic structure calculations and rovibrational spectra

Various ab initio methods are used to compute the six dimensional potential energy surfaces (6D-PESs) of the ground states of the H2NSi and H2SiN radicals. They include standard coupled cluster (RCCSD(T)) techniques and the newly developed explicitly correlated RCCSD(T)-F12 methods. For H2NSi, the explicitly correlated techniques are viewed to provide data as accurate as the standard coupled cluster techniques, whereas small differences are noticed for H2SiN. These PESs are found to be very flat along the out-of-plane and some in-plane bending coordinates. Then, the analytic representations of these PESs are used to solve the nuclear motions by standard perturbation theory and variational calculations. For both isomers, a set of accurate spectroscopic parameters and the vibrational spectrum up to 4000 cm-1 are predicted. In particular, the analysis of our results shows the occurrence of anharmonic resonances for H2SiN even at low energies. © 2011 American Institute of Physics.L.J. thanks the Centre National de la Recherche Scientifique (CNRS, France) for financial support. M.L.S. acknowledges the MICINN (SPAIN) for the Grant No. AYA2008– 00446.Peer Reviewe

A harmonic adiabatic approximation to calculate vibrational states of ammonia

In the present work, we study the vibrational spectrum of ammonia in full dimensionality (6D), with special emphasis on the tunneling splitting. The inversion motion, i.e., the active mode v(2), is indeed relatively well decoupled from the other five inactive modes. Therefore, an adiabatic separation in an active wave function and an inactive one, approximated with a 5D-harmonic basis function, is certainly well adapted to describing this motion. This separation leads to several 1D-effective Hamiltonians for each 5D-harmonic basis function or adiabatic channel. Two models have been tested: the harmonic adiabatic approximation (HADA), when only one channel is used and the coupled HADA (cHADA), when several channels are coupled. In order to get reliable values for tunneling splitting, our calculations have shown that: (i) the calculation of the electronic potential has to be performed with a large atomic basis set (up to quintuple zeta) with a method including core-valence correlation; (ii) the cHADA is required since the HADA overestimates the energy levels. Furthermore, our values for the tunneling splitting are in good agreement with the experimental data of ammonia and several isotopomers. (C) 2004 Elsevier B.V. All rights reserved

Reactive scattering of highly vibrationally excited oxygen molecules: Ozone formation?

A new ab initio potential energy surface based on an internally contracted multireference configuration-interaction wave function is constructed for the O2(X 3Σ-g,ν)+O2(X 3Σ-g,ν=0) →O3(X 1A1)+O(3P) reaction with ν>20. The vibrational state-to-state reaction probabilities are calculated with a time independent reactive scattering method. The state selected reactive rate constants calculated with 2D reduced dimensionality theory are very small, suggesting that the reaction of ozone formation is not significant in the O2(X 3Σ-g,ν)+O2(X 3Σ-g,ν=0) collision. © 1998 American Institute of Physics

THEORETICAL CALCULATION OF THE TORSIONAL SPECTRUM OF THE PARTIALLY DEUTERATED SPECIES OF METHANOL

$^{a}$Mukhopadhyay, Perry, Butler, Herbst, and De Lucia, 57th International Symposium on Molecular Spectroscopy, paper RH06 (2002) and Mukhopad-hyay, Perry, Lock, and Klee, 57th International Symposium on Molecular Spectroscopy, paper RH07 (2002). $^{b}$Lauvergnat and Nauts, J. Chem. Phys. 116, 8560 (2002). $^{c}$Light and Ba\u{c}i\'{c},J. Chem. Phys. 87, 408 (1987). $^{d}$Lauvergnat, Nauts, Justum and Chauisat, J. Chem. Phys. 114, 6592 (2001).Author Institution: Laboratoire de Chimie Physique, Unit\'{e} mixte du C.N.R.S. et de I'Universit\'{e} Paris-Sud,-B\^{a}t; Laboratoire de Photophysique Mol\'{e}cuiaire, C.N.R.S., B\^{a}t. 350, Universit\'{e} Paris-SudThe first spectroscopic investigations of the partially deuterated species of methanol, $CH_{2}DOH$ and $CD_{2}HOH$, have shown that these species display a dense torsional $spectrum^{a}$ difficult to assign. With a view toward understanding these spectra, a theoretical calculation of their rotation-torsion energy levels has been undertaken aided by ab initio calculations. This calculation accounts for the complicated torsion-rotation interaction displayed by these molecules and is based on the following features:\begin{itemize}\item The kinetic energy part of the Hamiltonian is calculated numerically taking into account all 12 internal degrees of freedom of the molecules. The angle of internal rotation is treated as an active $coordinate.^{b}$ \item The Schr\""{o}dinger equation for the internal rotation is solved using Gaussian $quadrature.^{c}$ \item Internal degrees of freedom corresponding to the other small amplitude motions are treated using the harmonic approximation, for each value of the internal angle of rotation corresponding to the DVR $grid.^{d}$ \item The potential energy function of the molecule is obtained using ab initio calculations. \end{itemize} After making some reasonable assumptions for the dipole moment function, the last step of the calculation involves evaluating the intensity of the rotation-torsion transitions. In the paper we hope to be able to show plots of room-temperature absorption spectra