Search

International audience; We present a theoretical evaluation of collision induced effects on a few typical doublets in the nu_4 band of ammonia perturbed by argon.Quantum dynamical calculations performed on two NH_3-Ar potential energy surfaces provide pressure broadening and intradoublet generalized cross sections. From these calculations we derive thermally averaged values at various temperatures. The intradoublet coupling terms at room temperature are found to be in good agreement with available data in the literature. In addition, we study the speed dependence of the pressure broadening and intradoublet coupling coefficients. The former show a usual speed dependence, quite important, but the later show a weak speed dependence at least around 296 K and above.

International audience; The photodetachment spectrum of the nitrate anion (NO − 3) is simulated from first principles using wavepacket quantum dynamics propagation and a newly developed accurate full-dimensional fully coupled five state diabatic potential model. This model utilizes the recently proposed complete nuclear permutation inversion invariant artificial neural network diabatization technique [D. M. G. Williams and W. Eisfeld, J. Phys. Chem. A 124, 7608 (2020)]. The quantum dynamics simulations are designed such that temperature effects and the impact of near threshold detachment are taken into account. Thus, the two available experiments at high temperature and at cryogenic temperature using the slow electron velocity-map imaging technique can be reproduced in very good agreement. These results clearly show the relevance of hot bands and vibronic coupling between theX 2 A ′ 2 ground state and theB 2 E ′ excited state of the neutral radical. This together with the recent experiment at low temperature gives further support for the proper assignment of the ν 3 fundamental, which has been debated for many years. An assignment of a not yet discussed hot band line is also proposed.

International audience; We present fully ab initio calculations of second-overtone rovibrational line shapes of carbon monoxide perturbed by argon. The quantum mechanical scattering problem between CO and Ar is solved numerically for two different ab initio interaction potentials. We use the generalized Hess method to determine spectroscopic cross sections which describe the effect of collisions on each spectral line. Using these cross sections, we determine the line-shape parameters that we use to generate the Hartmann-Tran and speed-dependent billiard ball profiles. We compare the generated line shapes with high-quality experimental line profiles of five lines measured at five pressures between 0.01 and 1 atm. A subpercent agreement over the entire pressure range is obtained. Calculations for the P(9) line are used to inspect the effects of the two interaction potentials. The discrepancies for both the considered interaction potentials and the experiment are explained within the described theoretical framework. The presented results are the most accurate collisional line-shape calculations for a system with collision dynamics representative of atmospherically relevant species.

International audience; We present a procedure for generating the parameters of the Hartmann-Tran profile (HTP) from purely first principles calculations. Starting from an absorber-perturber interaction potential, we calculate S-matrices describing the effect of collisions on the absorbing molecule. We then use the generalized Hess method to calculate speed-dependent pressure shift and broadening parameters, and the complex Dicke parameter, ν opt , which accounts for such effects as the Dicke effect and correlations between dephasing and velocity-changing collisions. Based on these ab initio results, we derive the Hartmann-Tran profile parameters and evaluate the validity of the quadratic approximation of speed dependence and the hard-collision model of velocity-changing collisions adopted in the Hartmann-Tran profile. We also discuss the interpretation and speed dependence of ν opt. Finally, we evaluate the approximation scheme for temperature dependence of HTP line-shape parameters adopted in the 2016 edition of the HITRAN database.

International audience; Near-infrared spectroscopy of the C2H2–Ar, Kr complexes was performed in the spectral region overlapping the ν3/ν2 + ν4 + ν5 Fermi-type resonance of C2H2. The experiment was conducted along the HElium NanoDroplet Isolation (HENDI) technique in order to study the coupling dynamics between a floppy molecular system (C2H2–Ar and C2H2–Kr) and a mesoscopic quantum liquid (the droplet). Calculations were performed using a spectral element based close-coupling program and state-of-the-art 2-dimensional potential energy surfaces to determine the bound states of the C2H2–Ar and C2H2–Kr complexes and simulate the observed spectra. This furnished a quantitative basis to unravel how the superfluid and non-superfluid components of the droplet affect the rotation and the deformation dynamics of the hosted complex.

International audience; A recently developed scheme to produce high-dimensional coupled diabatic potential energy surfaces (PESs) based on artificial neural networks (ANNs) [D. M. G. Williams and W. Eisfeld, J. Chem. Phys. 149, 204106 (2019)] is tested for its viability for quantum dynamics applications. The method, capable of reproducing high-quality ab initio data with excellent accuracy, utilizes simple coupling matrices to produce a basic low-order diabatic potential matrix as an underlying backbone for the model. This crude model is then refined by making its expansion coefficients geometry-dependent by the output neurons of the ANN. This structure, strongly guided by a straightforward physical picture behind nonadiabatic coupling, combines structural simplicity with high accuracy, reproducing ab initio data without introducing unphysical artifacts to the surface, even for systems with complicated electronic structure. The properties of diabatic potentials obtained by this method are tested thoroughly in the present study. Vibrational/vibronic eigenstates are computed on theX andà states of NO 3 , a notoriously difficult Jahn-Teller system featuring strong nonadiabatic couplings and complex spectra. The method is investigated in terms of how consistently it produces dynamics results for PESs of similar (fitting) quality and how the results depend on the ANN size and ANN topography. A central aspect of this work is to understand the convergence properties of the new method in order to evaluate its predictive power. A previously developed, high-quality model utilizing a purely (high-order) polynomial ansatz is used as a reference to showcase improvements of the overall quality which can be obtained by the new method.

International audience; E ⊗ e Jahn-Teller (JT) systems are considered the prototype of symmetry-induced conical intersections and of the corresponding geometric phase effect (GPE). For decades, this has been analyzed for the most common case originating from C 3v symmetry and these results usually were generalized. In the present work, a thorough analysis of the JT effect, vibronic coupling Hamiltonians, GPE, and the effect on spectro-scopic properties is carried out for general Cnv symmetric systems (and explicitly for n = 3-8). It turns out that the C 3v case is much less general than often assumed. The GPE due to the vibronic Hamiltonian depends on the leading coupling term of a diabatic representation of the problem, which is a result of the explicit n, α, and β values of a Cnv Eα ⊗ e β system. Furthermore, the general existence of n/m (m ∈ N depending on n, α, and β) equivalent minima on the lower adiabatic sheet of the potential energy surface (PES) leads to tunneling multiplets of n/m states (state components). These sets can be understood as local vibrations of the atoms around their equilibrium positions within each of the local PES wells symmetrized over all equivalent wells. The local vibrations can be classified as tangential or radial vibrations, and the quanta in the tangential mode together with the GPE determine the level ordering within each of the vibronic multiplets. Our theoretical predictions derived analytically are tested and supported by numerical model simulations for all possible Eα ⊗ e β cases for Cnv symmetric systems with n = 3-8. The present interpretation allows for a full understanding of the complex JT spectra of real systems, at least for low excitation energies. This also opens a spectroscopic way to show the existence or absence of GPEs. Published under license by AIP Publishing. https://doi.

A new full-dimensional (6D) diabatic potential energy surface (PES) model is presented representing the five lowest PESs corresponding to the X 2 A 2 ′ , A 2 E ″ , and B 2 E ′ electronic states of the nitrate radical (NO3). It is based on high-level ab initio calculations of roughly 90000 energy data over a wide range of nuclear configurations and represents the energies with a root mean-squares (rms) error of about 100 cm−1. An accurate dipole surface was developed for the X state as well. The new PES model is used to re-investigate the infra-red (IR) spectrum corresponding to the electronic ground state by full dimensional quantum dynamics simulations. Vibrational eigenstates, IR transition probabilities, and isotopic shifts are computed and analyzed. Levels up to 2000 cm−1 are obtained and show good to excellent agreement with known experimental values. Some larger deviations are observed and discussed as well. The new results are in agreement with previous theoretical studies that the disputed ν 3 fundamental corresponds to a frequency of roughly 1022 cm−1 and that the prominent experimental feature observed at 1492 cm−1 is due to the 3 1 4 1 ( e ′ ) combination mode. Observed discrepancies in the IR intensities may be explained by coupling to the B state which is also analysed by diabatic decomposition of the eigenstates.

We apply the spectral element method to the determination of scattering and bound states of the multichannel Schr\"odinger equation. In our approach the reaction coordinate is discretized on a grid of points whereas the internal coordinates are described by either purely diabatic or locally diabatic (diabatic-by-sector) bases. Bound levels and scattering matrix elements are determined with spectral accuracy using relatively small numbers of points. The scattering problem is cast as a linear system solved using state-of-the-art sparse matrix non iterative packages. Boundary conditions can be imposed so to compute a single column of the matrix solution. A comparison with log-derivative propagators customarily used in molecular physics is performed. The same discretization scheme can also be applied to bound levels that are computed using direct scalable sparse-matrix solvers., Comment: 24 pages, 5 figures

The E-2 '' state of NO3, a prototype for the Jahn-Teller effect, has been an enigma and a challenge for a long time for both experiment and theory. We present a detailed theoretical study of the vibronic quantum dynamics in this electronic state, uncovering the effects of tunnelling, geometric phase, and symmetry. To this end, 45 vibronic levels of NO3 in the E-2 '' state are determined accurately and analyzed thoroughly. The computation is based on a high quality diabatic potential representation of the two-sheeted surface of the E-2 '' state developed by us [W. Eisfeld et al., J. Chem. Phys. 140, 224109 (2014)] and on the multi-configuration time dependent Hartree approach. The vibrational eigenstates of the NO3- anion are determined and analyzed as well to gain a deeper understanding of the symmetry properties of such D-3h symmetric systems. To this end, 61 eigenstates of the NO3- anion ground state are computed using the single sheeted potential surface of the (1)A(1) state published in the same reference quoted above. The assignments of both the vibrational and vibronic levels are discussed. A simple model is proposed to rationalize the computed NO3 spectrum strongly influenced by the Jahn-Teller couplings, the associated geometric phase effect, and the tunnelling. Comparison with the available spectroscopic data is also presented. Published by AIP Publishing.

We study theoretically magnetically induced Feshbach resonances and near-threshold bound states in isotopic NaK pairs. Our calculations accurately reproduce Feshbach spectroscopy data on Na$^{40}$K and explain the origin of the observed multiplets in the p-wave [Phys. Rev. A 85, 051602(R) (2012)]. We apply the model to predict scattering and bound state threshold properties of the boson-boson Na$^{39}$K and Na$^{41}$K systems. We find that the Na$^{39}$K isotopic pair presents broad magnetic Feshbach resonances and favorable ground-state features for producing non-reactive polar molecules by two-photon association. Broad s-wave resonances are also predicted for Na$^{41}$K collisions., Comment: 39 pages, 15 figures. Phys. Rev. A in press

International audience; An analytical potential energy surface for a rigid Rb₂ in the ³Σ(u)⁺ state interacting with one helium atom based on accurate ab initio computations is proposed. This 2-dimensional potential is used, together with the pair approximation approach, to investigate Rb₂ attached to small helium clusters He(N) with N = 1-6, 12, and 20 by means of quantum Monte Carlo studies. The limit of large clusters is approximated by a flat helium surface. The relative orientation of the dialkali axis and the helium surface is found to be parallel. Dynamical investigations of the pendular and of the in-plane rotation of the rigid Rb₂ molecule on the surface are presented.

In this paper effects of higher order Jahn-Teller coupling terms on the nonadiabatic dynamics are studied. Of particular interest is the case when the potential energy surfaces of the degenerate state show pronounced anharmonicity. In order to demonstrate the effects a two-dimensional E multiply sign in circle e Jahn-Teller model system is treated which is based on the e(') stretching vibration of the photoactive (2)E(') state of NO(3) as a realistic example. The sixth order E multiply sign in circle e Jahn-Teller Hamiltonian is derived in the diabatic representation which is valid for any system with a C(3) rotation axis. This diabatization scheme is compared to lower-order Jahn-Teller Hamiltonians and to symmetry adapted as well as ad hoc approximations. Lower-order potentials result in pronounced quantitative and qualitative differences in the dynamics, including differences in the evolution of mean values, the autocorrelation functions (and thus the corresponding spectra), and the electronic population evolution. In the particular example treated, the results of fourth and fifth order potentials are very similar to the sixth order reference system. In contrast, the approximate sixth order Hamiltonians, though the corresponding adiabatic surfaces seem to be nearly identical, results in pronounced differences. The possible consequences for the dynamics of realistic systems with higher dimensionality are briefly discussed.

We present a comparative study of the rotational characteristics of various molecule-doped 4 He clusters using quantum Monte Carlo techniques. The theoretical conclusions obtained from both zero and finite temperature Monte Carlo studies confirm the presence of two different dynamical regimes that correlate with the magnitude of the rotational constant of the molecule, i.e. fast or slow rotors. For a slow rotor, the effective rotational constant for the molecule inside the helium droplet can be determined by a microscopic two-fluid model in which helium densities computed by path integral Monte Carlo are used as input, as well as by direct computation of excited energy levels. For a faster rotor, the conditions for application of the two-fluid model for dynamical analysis are usually not fulfilled and the direct determination of excitation energies is then mandatory. Quantitative studies for three molecules are summarised, showing in each case excellent agreement with experimental results.

International audience; We present quantum scattering calculations of 4 He and 3 He colliding with 87 Rb 2. For both helium isotopes, the elastic and inelastic rate coefficients are strongly influenced by the J = 1 partial wave. For the lighter isotope a strong resonance feature of the J = 1 partial wave is responsible for an extremely efficient vibrational relaxation process. We also perform bound-state calculations of the Rb 2 He complex for even Rb permutation symmetry and non-zero total angular momentum. The global Rb 2 He 3 Σ + u potential energy surface used supports four bound states for 4 He and a single one for 3 He. We propose an analysis of the 87 Rb 2 4 He spectrum separating the contributions of Rb 2 rotation and helium motion.

International audience; A scheme to produce accurate full-dimensional coupled diabatic potential energy surfaces including dissociative regions and suitable for dynamical calculations is proposed. The scheme is successfully applied to model the two-sheeted surface of the 2E″ state of the NO3 radical. An accurate potential energy surface for the NO−3NO3− anion ground state is developed as well. Both surfaces are based on high-level ab initio calculations. The model consists of a diabatic potential matrix, which is expanded to higher order in terms of symmetry polynomials of symmetry coordinates. The choice of coordinates is key for the accuracy of the obtained potential energy surfaces and is discussed in detail. A second central aspect is the generation of reference data to fit the expansion coefficients of the model for which a stochastic approach is proposed. A third ingredient is a new and simple scheme to handle problematic regions of the potential energy surfaces, resulting from the massive undersampling by the reference data unavoidable for high-dimensional problems. The final analytical diabatic surfaces are used to compute the lowest vibrational levels of NO−3NO3− and the photo-electron detachment spectrum of NO−3NO3− leading to the neutral radical in the 2E″ state by full dimensional multi-surface wave-packet propagation for NO3 performed using the Multi-Configuration Time Dependent Hartree method. The achieved agreement of the simulations with available experimental data demonstrates the power of the proposed scheme and the high quality of the obtained potential energy surfaces.

International audience; This article aims to present a general method that enables one to build kinetic energy matrices in getting rid, for the angular coordinates (internal and Eulerian), of the heaviness of di †erential calculus (for expressing kinetic energy operators) and numerical integration (for calculating matrix elements). Therefore, instead of 3N [ 3 coordinates, only N [ 1 radial distances are to be treated as coordinates. In the present formulation, the system is described by any set of n vectors i \ 1,. .. , nN and the kinetic energy operator is expressed in MR i , term of (n [ 1) angular momenta i \ 1,. .. , n [ 1N and the total angular momentum J. The formalism ML i , proposed is general and gives a remarkably compact expression of the kinetic energy in terms of the angular momenta. This expression allows one to circumvent the seeming angular singularities.

International audience; Following our work on the study of helium droplets and film doped with one electronically excited rubidium atom Rb(∗) ((2)P) [M. Leino, A. Viel, and R. E. Zillich, J. Chem. Phys. 129, 184308 (2008)], we focus in this paper on the second excited state. We present theoretical studies of such droplets and films using quantum Monte Carlo approaches. Diffusion and path integral Monte Carlo algorithms combined with a diatomics-in-molecule scheme to model the nonpair additive potential energy surface are used to investigate the energetics and the structure of Rb(∗)He(n) clusters. Helium films as a model for the limit of large clusters are also considered. As in our work on the first electronic excited state, our present calculations find stable Rb(∗)He(n) clusters. The structures obtained are however different with a He-Rb(∗)-He exciplex core to which more helium atoms are weakly attached, preferentially on one end of the core exciplex. The electronic absorption spectrum is also presented for increasing cluster sizes as well as for the film.

International audience; Full-dimensional multiconfigurational time-dependent Hartree calculations on the tunneling splitting of the vibrational ground state and the low lying excited states of malonaldehyde are presented. Methodological developments utilizing the symmetry of double well systems for the efficient calculation of tunneling splittings are described and discussed. Important aspects of the theory underlying the previously communicated results for the ground state tunneling splitting [M. D. Coutinho-Neto et al., J. Chem. Phys. 121, 9207 (2004)] are detailed and further developments facilitating the calculation of tunneling splittings for vibrationally excited states are introduced. Utilizing these developments, the 14 lowest vibrational states of malonaldehyde, i.e., seven tunneling splittings, have been computed. The tunneling splittings are found to vary significantly depending on the particular vibrational excitation. This results in a complex pattern of vibrational levels. Studying the dependence of the tunneling splittings on the vibrational excitation, good agreement with available experimental results is found and intuitive interpretations of the results can be given.

International audience; We present theoretical studies of helium droplets and films doped with one electronically excited rubidium atom Rb( *) ((2)P). Diffusion and path integral Monte Carlo approaches are used to investigate the energetics and the structure of clusters containing up to 14 helium atoms. The surface of large clusters is approximated by a helium film. The nonpair additive potential energy surface is modeled using a diatomic in molecule scheme. Calculations show that the stable structure of Rb( *)He(n) consists of a seven helium atom ring centered at the rubidium, surrounded by a tirelike second solvation shell. A very different structure is obtained when performing a "vertical Monte Carlo transition." In this approach, a path integral Monte Carlo equilibration starts from the stable configuration of a rubidium atom in the electronic ground state adsorbed to the helium surface after switching to the electronically excited surface. In this case, Rb( *)He(n) relaxes to a weakly bound metastable state in which Rb( *) sits in a shallow dimple. The interpretation of the results is consistent with the recent experimental observations [G. Aubock et al., Phys. Rev. Lett. 101, 035301 (2008)].

Accurate quantum determination of the ground state tunneling splitting zero-point energy using the full dimensional potential proposed by Yagi et al. [J. Chem. Phys. 115:10647, 2001] are reported. Two exact quantum dynamics methods are used: the multi-configurational time-dependent Hartree (MCTDH) approach and the diffusion Monte Carlo based projection operator imaginary time spectral evolution (POITSE) method. In this report, we focus on the challenges faced by the MCTDH approach and the steps taken for obtaining the benchmark value. The MCTDH computation yields 25 cm−1 converged to about 10% accuracy while POITSE obtains a value for the tunneling splitting of 25.7±0.3 cm−1 which compares well with the experimental value of 21.6 cm−1. These rigorous results are used to evaluate the accuracy of approximate dynamical approaches, e.g. the instanton theory.

The determination of the photoelectron spectrum of NH3 and of the internal conversion dynamics of NH3+ recently published [A. Viel, W. Eisfeld, S. Neumann, W. Domcke, U. Manthe, J. Chem. Phys. 124 (2006) 214306] is complemented by the investigation of the effect of the vibrational angular momenta couplings on the dynamics. The multi-configurational time-dependent Hartree method is used to propagate a wave packet on the analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation. Curvilinear coordinates and the associated quasi-exact kinetic energy operator suitable for the multi-configurational time-dependent Hartree scheme are employed. A non-negligible effect of the use of Cartesian normal modes instead of the curvilinear coordinates is observed on the low energy part of the photoelectron spectrum. However, the three different time scales found in the dynamical calculations for the second absorption band are very similar regardless of the use of either normal modes or curvilinear coordinates. (C) 2007 Elsevier B.V. All rights reserved.

International audience; In this work we present a theoretical and experimental study of the acetylene–hydrogen system. A potential surface considering rigid monomers has been obtained by ab initio quantum chemistry methods. This 4-dimensional potential is further employed to compute, using the close-coupling approach and the coupled-states approximation, pressure broadening coefficients of C2H2 isotropic Raman Q lines over a temperature range of 77 to 2000 K. Experimental data for the acetyleneν2 Raman lines broadened by molecular hydrogen are obtained using stimulated Raman spectroscopy. The comparison of theoretical values with experimental data at 143 K is promising. Approximations to increase the computational efficiency are proposed.

We present diffusion Monte Carlo calculations of the ground and first excited vibrational states of NH(3) (4)He(n) for n< or =40. We use the potential energy surface developed by one of us [M. P. Hodges and R. J. Wheatley, J. Chem. Phys. 114, 8836 (2001)], which includes the umbrella mode coordinate of NH(3). Using quantum Monte Carlo calculations of excited states, we show that this potential is able to reproduce qualitatively the experimentally observed effects of the helium environment, namely, a blueshift of the umbrella mode frequency and a reduction of the tunneling splittings in ground and first excited vibrational states of the molecule. These basic features are found to result regardless of whether dynamical approximations or exact calculations are employed.

International audience; Quantum dynamics calculations of the ground state tunneling splitting and of the zero point energy of malonaldehyde on the full dimensional potential energy surface proposed by Yagi et al. [J. Chem. Phys. 1154, 10647 (2001)] are reported. The exact diffusion Monte Carlo and the projection operator imaginary time spectral evolution methods are used to compute accurate benchmark results for this 21-dimensional ab initio potential energy surface. A tunneling splitting of 25.7+/-0.3 cm-1 is obtained, and the vibrational ground state energy is found to be 15 122+/-4 cm-1. Isotopic substitution of the tunneling hydrogen modifies the tunneling splitting down to 3.21+/-0.09 cm-1 and the vibrational ground state energy to 14 385+/-2 cm-1. The computed tunneling splittings are slightly higher than the experimental values as expected from the potential energy surface which slightly underestimates the barrier height, and they are slightly lower than the results from the instanton theory obtained using the same potential energy surface.

An analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation has been developed which is tailored to model the ultrafast photoinduced dynamics. Selected ab initio cuts, obtained by multireference configuration interaction calculations, have been used to determine the parameters of a diabatic representation for this Jahn-Teller and pseudo-Jahn-Teller system. The model includes higher-order coupling terms both for the Jahn-Teller and for the pseudo-Jahn-Teller matrix elements. The relaxation to the ground state is possible via dynamical pseudo-Jahn-Teller couplings involving the asymmetric bending and stretching coordinates. The photoelectron spectrum of NH3 and the internal conversion dynamics of NH+3 have been determined by wave packet propagation calculations employing the multiconfigurational time-dependent Hartree method. Three different time scales are found in the dynamics calculations for the second absorption band. The ultrafast Jahn-Teller dynamics of the two excited states occurs on a 5 A time scale. The major part of the internal conversion to the ground state takes place within a short time scale of 20 fs. This fast internal conversion is, however, incomplete and the remaining excited state population does not decay completely even within 100 fs. (c) 2006 American Institute of Physics.

The pseudo-Jahn-Teller (PJT) coupling of a nondegenerate state A with a twofold degenerate state E by a degenerate vibrational mode e is studied for a general system with a C(3) main rotational axis. The PJT coupling terms up to sixth order are derived by symmetry considerations for this general (A+E) multiply sign in circle e case. The obtained expression for the 3 x 3 diabatic potential energy matrix is found to be closely related to the expression recently developed for the higher order Jahn-Teller case [A. Viel and W. Eisfeld, J. Chem. Phys. 120, 4603 (2004)]. The dynamical PJT coupling, which can arise for states of appropriate symmetry if one of the vibrational modes induces a change of the nuclear point group between D(3h), C(3v), C(3h), and C(3), is discussed. The effect of the higher order PJT coupling is tested by a two-dimensional model study based on the e bending mode of NH(3)(+). The models are analyzed by fitting the two-dimensional potential energy surfaces. The significance of the higher order terms on the nonadiabatic dynamics is demonstrated by quantum wave packet propagations.

Benchmark calculations of the tunneling splitting in malonaldehyde using the full dimensional potential proposed by Yagi [J. Chem. Phys. 115, 10647 (2001)] are reported. Two exact quantum dynamics methods are used: the multiconfigurational time-dependent Hartree (MCTDH) approach and the diffusion Monte Carlo based projection operator imaginary time spectral evolution (POITSE) method. A ground state tunneling splitting of 25.7+/-0.3 cm(-1) is calculated using POITSE. The MCTDH computation yields 25 cm(-1) converged to about 10% accuracy. These rigorous results are used to evaluate the accuracy of approximate dynamical approaches, e.g., the instanton theory. (C) 2004 American Institute of Physics. (C) 2004 American Institute of Physics.

International audience

International audience; The effects of including rotational degrees of freedom on helium solvation densities in molecule-doped helium clusters are investigated for a variety of molecules. Helium densities and cluster energetics are calculated with diffusion Monte Carlo methods. The rotationally induced changes in the helium density distributions are examined and quantified with a theoretical estimator applicable to molecules of arbitrary symmetry. This analysis leads to a discussion of adiabatic following of molecular rotation in a solvating helium environment. We make a detailed comparative study of the effect of molecular rotation as a function of four impurity molecules with varying mass and symmetry: SF6, OCS, HCN, and benzene (C6H6). We find that even for the heaviest rotors, only a fraction of the solvating helium density adiabatically follows the molecular motion in the quantum ground state. For the lightest molecule, HCN, a negligible degree of adiabatic following is found. A discussion of the various definitions is presented to clarify the meaning of adiabatic following, and its applicability to dynamical models of quantum rotation in helium droplets is evaluated in light of the quantitative findings of incomplete adiabatic following established here.

International audience; A six-dimensional analytic potential-energy surface of the three valence states (N,V, Z) of ethene has been constructed on the basis of complete-active-space ab initio calculations and ab initio calculations with perturbation theory of second order based on a complete active reference space. The nuclear coordinate space is spanned by the torsion, the C–C stretch coordinate, the left and right pyramidalization and the symmetric and antisymmetric scissor coordinates. The C–H stretch coordinates and the CH2 rocking angles are kept frozen at their ground-state equilibrium value. A diabatic representation of the valence states of ethene has been constructed within the framework of a Hückel-type model. The diabatic potential-energy elements are represented as analytic functions of the relevant coordinates. The parameters of the analytic functions have been determined by a least-squares fit of the eigenvalues of the diabatic potential-energy matrix to the ab initio data for one-dimensional and two-dimensional cuts of the six-dimensional surface. As a function of the torsion, the analytic potential-energy surface describes the intersections of the V and Z states for torsional angles near 90°, which are converted into conical intersections by the antisymmetric scissor mode. As a function of pyramidalization of perpendicular ethene, it describes the intersections of the diabatic N and Z states, which are converted into conical intersections by displacements in the torsional mode. The analytic potential-energy surfaces can provide the basis for a quantum wave packet description of the internal conversion of photoexcited ethene to the electronic ground state via conical intersections.

Rigorous three‐dimensional quantum mechanical calculations have been carried out for the cumulative reaction probability of the titled reaction for total angular momentum, J=0. Higher values of J are included approximately, and the resulting thermal rate constant agrees well with experimental values for 800≤T≤3000 K.

We present an algorithm for rigid body diffusion Monte Carlo with importance sampling, which is based on a rigorous short-time expansion of the Green's function for rotational motion in three dimensions. We show that this short-time approximation provides correct sampling of the angular degrees of freedom, and provides a general way to incorporate importance sampling for all degrees of freedom. The full importance sampling algorithm significantly improves both calculational efficiency and accuracy of ground state properties, and allows rotational and bending excitations in molecular van der Waals clusters to be studied directly., Comment: Accepted for publication in Computer Physics Communications

International audience; We present a new method adapted to the calculation of excited rovibrational states of semirigid molecules. It first relies on a description of the molecule in terms of polyspherical coordinates of Jacobi vectors, in order to obtain a compact expression for the kinetic energy operator T̂(q). This general description is then adapted to the molecule considered by defining curvilinear normal modes from the corresponding zero order harmonic Hamiltonian Ĥ0=T̂(qeq) + Vharm(q), the solutions of which are being used as the working basis set. The residual kinetic term ΔT̂ is treated mainly analytically in this basis, and displays no radial contribution. Anharmonic coupling ΔV(q) is handled by means of a pseudospectral scheme based on Gauss Hermite quadratures. This method is particularly adapted to direct iterative approaches which only require the action of Ĥ on a vector, without the need of the associated matrix, thus allowing ultralarge bases to be considered. An application to the excited vibrational states of the HFCO molecule is presented. It is shown in this example that energy levels can be trivially assigned from the leading expansion coefficient of the associated eigenvector.

International audience; Bound energy levels, up to 5000 cm−1 of internal excitation, have been computed for the HFCO molecule. An exact six-dimensional (6D) quantum Hamiltonian, expressed in terms of Jacobi vectors, has been used. It is shown to lead to a compact form of the kinetic energy operator, easy to implement in the calculations. The primary discrete variable representation (DVR) basis set has been contracted by means of the adiabatic pseudospectral method of Friesner et al. [J. Chem. Phys. 99, 324 (1993)]. Two different, global, potential energy surfaces have been considered. The calculated energy levels have been successfully assigned by an automatic labeling procedure. These levels have been compared to the experimental results, providing a test of the accuracy of the existing surfaces.

Parallel implementation of an iterative determination of energy levels of large molecular systems is presented. The basic step consists in acting the Hamiltonian operator H on a wavefunction u , and is achieved by means of a Pseudo Spectral Split Hamiltonian scheme (Leforestier et al., J. Chem. Phys. 106 (1997) 8527). The potential term evaluation V . u , which corresponds by far to the most time consuming part in the sequential code, has been distributed over all the processors. Application to the water dimer (H 2 O) 2 Vibration–Rotation spectrum shows a very good parallel efficiency up to 64 processors.

International audience; Quantum rate calculations for the H+O2→HO+OH+O2→HO+O combustion reaction, as well as for the reverse reaction, are reported. Using the DMBE IV potential energy surface, the cumulative reaction probability N0(E)N0(E) has been directly computed for total angular momentum J=0, by means of the Seideman, Manthe and Miller Lanczos-based absorbing boundary condition method [J. Chem. Phys. 96, 4412 (1992); 99, 3411 (1993)]. Special attention has been paid to the definition of the molecular basis set, and to the sensitivity of the results to the absorbing potentials used in the asymptotic regions. The N0(E) results show very good overall agreement with the coupled channel calculations of Pack et al. [J. Chem. Phys. 102, 5998 (1995)], although the highly oscillatory behavior just above threshold renders such a comparison difficult in that energy range. The behavior of the J≠0 cumulative reaction probability has been estimated from calculations using the Jz-conserving approximation for J values in the range 10–70. This allowed us to define which reference geometry should be used in the J-shifting approximation, in order to compute the cumulative reaction probability NJ(E) for any J value. By imposing conservation of the total energy within this approximation, the rate constants are shown to display better agreement with the experimental results.

International audience; We have developed a three-dimensional potential energy surface for the lowest triplet state of the Rb2He complex. A global analytic fit is provided as in the supplementary material [see supplementary material at http://dx.doi.org/10.1063/1.4709433E-JCPSA6-136-034218 for the corresponding Fortran code]. This surface is used to perform quantum scattering calculations of 4He and 3He colliding with 87Rb2 in the partial wave J = 0 at low and ultralow energies. For the heavier helium isotope, the computed vibrational relaxation probabilities show a broad and strong shape resonance for a collisional energy of 0.15 K and a narrow Feshbach resonance at about 17 K for all initial Rb2 vibrational states studied. The broad resonance corresponds to an efficient relaxation mechanism that does not occur when 3He is the colliding partner. The Feshbach resonance observed at higher collisional energy is robust with respect to the isotopic substitution. However, its effect on the vibrational relaxation mechanism is faint for both isotopes.

International audience; The X(2)Σ ground and the A(2)Π and B(2)Σ first two excited states of Li-He and Na-He are determined using high level complete active space self-consistent field-multireference configuration interaction ab initio method. The obtained potentials differ from the ones proposed by Pascale [Phys. Rev. A 28, 632 (1983)], more strongly for the ground than for the excited states. Quantum diffusion Monte Carlo studies of small Li(∗)He(n) and Na(∗)He(n) with n ≤ 5 are performed using a diatomics-in-molecule approach to model the non-pair additive interaction potential. The sensitivity of our results to the A(2)Π and B(2)Σ potentials used is assessed by an analysis of the structure and of the energetics of the clusters. For these small clusters, the physical conclusions are essentially independent of the diatomic curves employed.

International audience; We present diffusion Monte Carlo calculations of ground states and rotationally excited states of HCN 4Hen,4Hen, using our recently developed algorithm for importance sampled rigid body diffusion Monte Carlo [Viel et al., Comput. Phys. Commun. (in press, 2001)] within the mixed frame implementation. Excited states are studied with both fixed node approximations, and the Projection Operator Imaginary Time Spectral Evolution (POITSE) method that allows nodal constraints to be circumvented. Improvements in the POITSE algorithm allow excited states of clusters with up to 80 degrees of freedom to be determined here. The results presented here show that the rotational dynamics of the HCN molecule in 4He4He clusters are very different from the behavior of heavier molecules such as SF6.SF6. Detailed analysis of ground state densities shows that the lighter HCN molecule induces negligible adiabatic following of the helium density as a result of its rotational motion. The excited state calculations show that for small numbers of 4He4He atoms the nodal structure does not correspond to that of a freely rotating molecule. Nevertheless, the POITSE calculations indicate that there is some admixture of this nodal structure in the low-lying rotational excitations. It is found that a relatively large number of 4He4He atoms are required to achieve saturation of the effective rotational constant at the experimental value, in contradistinction to the small numbers of atoms required to saturate the rotational constant for heavier molecules such as SF6SF6 and OCS.