Ph. Halvick

Analytical representations of high level ab initio potential energy curves of the C2 molecule

Abstract Realistic analytical representations of the twelve lowest singlet and triplet electronic adiabatic potential energy curves of C2 molecule are given in this article. The corresponding electronic states are correlated with C atoms both in their 3P state. A new set of high level MRCI calculations coupled with a double many-body expansion analytical fitting based on the extended Hartree–Fock approximate correlation energy model have been used in this work. Using RKR data available in the literature, comparison is made between our results and RKR turning points concerning the four lowest singlet states X1Σ+g, A1Πu, B1Δg and B′1Σ+g of C2. The agreement is very satisfying.

Visible emission from the vibrationally hot C2H radical following vacuum-ultraviolet photolysis of acetylene: Experiment and theory

Photolysis of acetylene has been performed by vacuum-ultraviolet excitation with the synchrotron radiation via the Rydberg states converging to the first ionization potential (IP) at 11.4 eV. Only the visible fluorescence of the ethynyl radical was observed in the A 2Π–X 2Σ+ system. Excitation of several Rydberg states of acetylene over a large energy range between 9 and 11.4 eV allowed us to observe for the first time the evolution of this continuum with increasing Rydberg excitation. Intensity calculations based on accurate ab initio potential energy surfaces of C2H were performed by using a one-dimensional model accounting for the large-amplitude motion of the H atom around the C–C bond and for the overall rotation of the radical. These calculations successfully reproduce the observed visible continuum (maximum at 500 nm and blue side cutoff at 400 nm) and bring new information on the distribution of the internal energy deposited in the fragment. For most excited Rydberg states, predissociation occurs...

Low temperature quantum rate coefficient of the H + CH+ reaction.

In this paper we report the first theoretical study of the title reaction. A global, single-valued model of the ground-state potential energy surface has been obtained by fitting to an extensive set of high-level ab initio calculations. The surface is found to be attractive apart from linear geometries where energy barriers appear due to conical intersections. This model was then used to calculate the reactive reactant state selected cross sections for collision energies ranging from threshold up to 4000 cm(-1). These calculations were performed using our version of the Baers approach of the RIOSA-NIP method which is based on the use of a negative imaginary potential. We find that the reaction probability is extremely oscillatory as a function of kinetic energy as it is a case for insertion reactions with a low exoergicity. The resulting reaction rate coefficient is found to first increase slowly as a function of temperature up to a broad maximum around 20 K and then to decrease slowly when temperature keeps increasing.

A theoretical study of acetylene: toward the complete characterization of the singlet ground state potential energy surface

Abstract The singlet ground state potential energy surface of acetylene has been thoroughly explored up to 43000 cm −1 by using high level of ab initio molecular orbital theory, including correlation effects at fourth order perturbation theory. Geometries, energies and frequencies for eight stationary points, and the minimum energy paths connecting them are calculated. These ab initio informations are used to build the topological skeleton for a model of potential energy surface. We check the topological consistency of this model with a visual approach and with the Morse theory.

Rotational relaxation of HF by collision with ortho- and para-H2 molecules

Quantum mechanical investigation of the rotationally inelastic collisions of CS with ortho- and para-H2 molecules is reported. The new global four-dimensional potential energy surface presented in our recent work is used. Close coupling scattering calculations are performed in the rigid rotor approximation for ortho- and para-H2 colliding with CS in the j = 0-15 rotational levels and for collision energies ranging from 10(-2) to 10(3) cm(-1). The cross sections and rate coefficients for selected rotational transitions of CS are compared with the ones previously reported for the collision of CS with He. The largest discrepancies are observed at low collision energy, below 1 cm(-1). Above 10 cm(-1), the approximation using the square root of the relative mass of the colliders to calculate the cross sections between a molecule and H2 from the data available with (4)He is found to be a good qualitative approximation. The rate coefficients calculated with the electron gas model for the He-CS system show more discrepancy with our accurate results. However, scaling up these rates by a factor of 2 gives a qualitative agreement.

Ab initio study of the potential energy surfaces for the reaction N(4Su +CH(X 2IIr) → CN(X 2Σ+, A 2IIi + H(2Sg)

Abstract The three lowest triplet potential energy surfaces of HCN have been investigated with the complete active space multiconfigurational self consistent field (CAS/MCSCF) method. Two surfaces are issued from the ground state reactants, while the third one is issued from the first excited state of the reactants, and is coupled to the others by a conical intersection. Of the two surfaces issued from the ground state reactants, one possesses a very small potential barrier in the entrance channel, while the other surface has none, thus making the reaction possible at very low temperature. For each surface, two potential wells have been found, corresponding to bent structures HCN and HNC. These potential wells are separated by high isomerisation saddle points. The approach of H toward CN shows a potential barrier for each of the three surfaces. Some parts of the conical intersection have been calculated and found energetically lower than the ground state reactants. Thus, transitions between surfaces should play an important role in the reaction dynamics.

Accurate global potential energy surface for the H + OH+ collision

We mapped the global three-dimensional potential energy surface (3D-PES) of the water cation at the MRCI/aug-cc-pV5Z including the basis set superposition (BSSE) correction. This PES covers the molecular region and the long ranges close to the H + OH(+)(X(3)Σ(-)), the O + H2(+)(X(2)Σg(+)), and the hydrogen exchange channels. The quality of the PES is checked after comparison to previous experimental and theoretical results of the spectroscopic constants of H2O(+)(X(2)B1) and of the diatomic fragments, the vibronic spectrum, the dissociation energy, and the barrier to linearity for H2O(+)(X(2)B1). Our data nicely approach those measured and computed previously. The long range parts reproduce quite well the diatomic potentials. In whole, a good agreement is found, which validates our 3D-PES.

Coupled ab initio potential energy surfaces for the two lowest 2A′ electronic states of the C2H molecule

Realistic two-valued potential energy surfaces for the reaction C(3P) + CH(X2Π) → C2 + H have been constructed from a set of high level ab initio data describing the first two 2A′ electronic states of the C2H system. These states have linear equilibrium configurations, known as the X 2Σ+ and A2Π states, and are coupled by a conical intersection. They lead to the formation of C2(X1Σ+ g) and C2(a3Πu) considering an adiabatic dissociation process. The ab initio calculations are of the multireference configuration interaction variety and were carried out using a polarized triple-zeta basis set. Using the ab initio adiabatic energies and the matrix elements of the dipole moment, a 2 × 2 diabatic representation of the electronic Hamiltonian was built. Each element of this Hamiltonian matrix was expressed within the double many-body expansion (DMBE) scheme which is based, in this case, on the extended Hartree-Fock approximate correlation energy model (EHFACE). The analytical adiabatic potential energy surfaces are then obtained as the eigenvalues of this matrix, and display correctly the Σ/Π conical intersection. Moreover, the non-adiabatic couplings given by our analytical model are compared with the ab initio ones, and good qualitative agreement is observed.

Theoretical spectroscopic characterization of the ArBeO complex

Using the recently developed explicitly correlated coupled cluster method in connection with the aug-cc-pVTZ basis set, we generated the three-dimensional potential energy surface (3D-PES) of the ground state of the Ar-BeO complex. This PES covers the regions of the global and local minima, the saddle point, and the dissociation of the complex. The PES is also used for the calculation of the rovibrational spectrum up to the dissociation limit. The high density of levels which is observed favors the mixing of the states and hence the occurrence of anharmonic resonances. The wavefunctions of the high rovibrational levels exhibit large amplitude motions in addition to strong anharmonic resonances. Our theoretical spectrum should be helpful in identifying the van der Waals modes of this complex in laboratory.

Trajectory surface hopping study of the C + CH reaction

The influence of electronically nonadiabatic transitions in the C(3Pg) + CH(X 2Π) → C2(X 1Σ+g, a 3Πu) + H(2Sg) reaction is investigated by using Tullys fewest-switches version of the trajectory surface hopping method. A diabatic model of the first two 2A′ potential energy surfaces coupled by a conical intersection is used. The diatomic CH has the internal state (ν = 0, j = 0) and batches of 20 000 trajectories are computed for four collision energies, E = 0.1, 0.3, 0.5 and 0.7 eV. We find that the reaction dynamics does not exhibit a statistical character, despite the existence of deep wells along the reaction path. Only the distribution of scattering angle shows a good agreement between trajectories and phase space theory results. A strong excess of vibrational energy is disposed on both electronic products C2(X 1Σ+g) and C2(a 3Πu), correlated with a lack of recoil energy. With all trajectories starting on a single potential surface, we obtain an electronic branching ratio X : a close to 2 : 3, only slightly dependent on the collision energy.