Céline Léonard

Time-dependent wave packet study on trans-cis isomerization of HONO driven by an external field

The present paper is devoted to a full quantum mechanical study of the cis-->trans isomerization of HONO. In contrast to our previous study [Richter et al., J. Chem. Phys. 120, 6072 (2004)], the dynamics is now performed in the presence of an external time-dependent field in order to be closer to experimental conditions. A six-dimensional dipole surface is computed. Using a previously developed potential energy surface [Richter et al., J. Chem. Phys. 120, 1306 (2004)], all eigenstates up to 4000 cm(-1) are calculated. We simulate the dynamics during and after excitation by an electromagnetic pulse whose parameters are chosen to efficiently trigger the isomerization. Our investigations show that there is a selective isomerization pathway.

The barrier to inversion of ammonia

A six-dimensional analytic potential energy surface for NH3 has been obtained using ab initio CCSD(T) data. The variational method has been used to determine its vibrational energy levels. The parameters in the surface which describe inversion have been refined such that the vibrational levels for inversion give best agreement with observation. Nearly all the fundamentals and overtones of the refined surface then agree with observation to within 1 cm -1. The barrier to inversion is found to be 1769 cm- 1. This is compared with other computed estimates of 1766 and 1777 cm -1 which appear in the literature.

Potential energy function and vibrational states of N2CO

A six-dimensional potential energy function (PEF) for the electronic ground state of N2CO+ (X 2A′) has been generated by electronic structure calculations using the restricted open shell coupled cluster RCCSD(T) approach. The ion has a planar trans equilibrium structure with: RNN=1.106 A, RNC=1.905 A, RCO=1.127A, θNNC=175.3°, and θNCO=129.1°. Variational calculations of the vibrational states (J=0) have yielded the following anharmonic wavenumbers for the fundamentals: ν1 (NN stretch) 2287.2, ν2 (CO stretch) 2071.0, ν3 (CN stretch+NCO bend) 546.9, ν4 (CN stretch+NCO bend) 215.2, ν5 (in-plane NNC bend) 123.4, ν6 (out-of-plane NNC bend) 133.8 (all values are in cm−1). For fixed equilibrium coordinates except one, the barriers to linearity have been calculated to be 8 cm−1 for the NNC and 2260 cm−1 for the NCO moieties, the torsional barrier to be 35 cm−1. It has been found that the ν3 and ν4 modes are strongly coupled, the in-plane ν5 and out-of-plane ν6 bending modes possess an inverse anharmonicity and fa...

New study of the stability and of the spectroscopy of the molecular anions NCO- and CNO-.

Using highly correlated wave functions, the ground and the low lying excited states of the molecular NCO(-) and CNO(-) anions have been reinvestigated. The stability of the electronic ground state of the two isomers with respect to dissociation and to electron detachment has been checked along the isomerization pathway. The regions of stability of the excited electronic states have been analyzed and identified and it is shown that only the ground state is stable and the corresponding potential energy surface presents three equilibrium positions. The rovibronic spectroscopy of the X (1)Σ(+) state of both NCO(-) and CNO(-) isomers has been determined by a variational approach leading to remarkable agreement with experimental data.

Renner–Teller effect in linear tetra-atomic molecules. I. Variational method including couplings between all degrees of freedom on six-dimensional potential energy surfaces

For electronically degenerate states of linear tetra-atomic molecules, a new method is developed for the variational treatment of the Renner-Teller and spin-orbit couplings. The approach takes into account all rotational and vibrational degrees of freedom, the dominant couplings between the corresponding angular momenta as well as the couplings with the electronic and electron spin angular momenta. The complete rovibrational kinetic energy operator is expressed in Jacobi coordinates, where the rovibrational angular momenta J(N) have been replaced by J-L(ez)-S and the spin-orbit coupling has been described by the perturbative term A(SO) x L(ez)S(z). Attention has been paid on the electronic wave functions, which require additional phase for linear tetra-atomic molecules. Our implemented rovibrational basis functions and the integration of the different parts of the total Hamiltonian operator are described. This new variational approach is tested on the electronic ground state X (2)Pi(u) of HCCH(+) for which new six-dimensional potential energy surfaces have been computed using the internally contracted multireference configuration interaction method and the cc-pV5Z basis set. The calculated rovibronic energies and their comparisons with previous theoretical and experimental works are presented in the next paper.

Theoretical investigation of intramolecular vibrational energy redistribution in HFCO and DFCO induced by an external field.

The present paper is devoted to a full quantum mechanical study of the intramolecular vibrational energy redistribution in HFCO and DFCO. In contrast to our previous studies [Pasin et al., J. Chem. Phys. 124, 194304 (2006) and 126, 024302 (2007)], the dynamics is now performed in the presence of an external time-dependent field. This more closely reflects the experimental conditions. A six-dimensional dipole surface is computed. The multiconfiguration time-dependent Hartree method is exploited to propagate the corresponding six-dimensional wave packets. Special emphasis is placed on the excitation of the out-of-plane bending vibration and on the dissociation of the molecule. In the case of DFCO, we predict that it is possible to excite the out-of-plane bending mode of vibration and to drive the dissociation to DF+CO with only one laser pulse with a fixed frequency and without excitation of an electronic state.

New Variational Method for the Ab Initio Study in Valence Coordinates of the Renner-Teller Effect in Tetra-Atomic Systems.

A new variational methodology for the treatment of the Renner-Teller effect in tetra-atomic molecules has been developed in valence coordinates. The kinetic-energy operator of Bramley et al. [Mol. Phys. 1991, 73, 1183] for any sequentially bonded four-atom molecule, A-B-C-D, in the singlet nondegenerate electronic state has been adapted to the Renner-Teller and spin couplings by modifying the expression of the nuclear angular momentum. The total Schrödinger equation is solved by diagonalizing the Hamiltonian matrix in a three-step contraction scheme. The main advantage of this new theoretical development is the possibility of studying different isotopomers using the same potential-energy surfaces. This procedure has been tested on HCCH(+) and its deuterated derivatives DCCD(+) and DCCH(+). The calculated rovibronic band origins were compared with previous data deduced from the Jacobi coordinates methodology, dimensionality reduced variational treatment, and photoelectron spectra with a good global agreement. Rotational structures for these systems are also tackled.

The selective population of the vibrational levels of thioformaldehyde

The simulation of the selective excitation of the vibrational levels of the electronic ground state of H2CS by femtosecond coherent light pulses was investigated. The simulation model was based on the development of the time-dependent wavefunction in terms of the stationary vibrational eigenstates. The dipole moment and the potential energy functions were obtained ab initio and approximated by polynomial expansions. Chirped and non-chirped linear polarized pulses were tried as well as circularly polarized pulses. If the target vibrational level was not involved in resonances a selectivity of more than 60% was achieved. In several cases the target state was 100% populated. Different ways of achieving the population were investigated.

Argon Interaction with Gold Surfaces: Ab Initio-Assisted Determination of Pair Ar–Au Potentials for Molecular Dynamics Simulations

Global potentials for the interaction between the Ar atom and gold surfaces are investigated and Ar-Au pair potentials suitable for molecular dynamics simulations are derived. Using a periodic plane-wave representation of the electronic wave function, the nonlocal van-der-Waals vdW-DF2 and vdW-OptB86 approaches have been proved to describe better the interaction. These global interaction potentials have been decomposed to produce pair potentials. Then, the pair potentials have been compared with those derived by combining the dispersionless density functional dlDF for the repulsive part with an effective pairwise dispersion interaction. These repulsive potentials have been obtained from the decomposition of the repulsive interaction between the Ar atom and the Au2 and Au4 clusters and the dispersion coefficients have been evaluated by means of ab initio calculations on the Ar+Au2 complex using symmetry adapted perturbation theory. The pair potentials agree very well with those evaluated through periodic vdW-DF2 calculations. For benchmarking purposes, CCSD(T) calculations have also been performed for the ArAu and Ar+Au2 systems using large basis sets and extrapolations to the complete basis set limit. This work highlights that ab initio calculations using very small surface clusters can be used either as an independent cross-check to compare the performance of state-of-the-art vdW-corrected periodic DFT approaches or, directly, to calculate the pair potentials necessary in further molecular dynamics calculations.

Large amplitude vibrations in the X 2A1 state of C2B

A three-dimensional potential energy function (PEF) of the 2A1 electronic ground state of C2B has been generated by electronic structure calculations. The PEF possesses a minimum in an isosceles triangular structure which lies 2204 cm−1 below two equivalent minima having linear equilibrium geometry. The barrier height between the minima relative to the triangular structure has been calculated to the 2383 cm−1. The nuclear motion problem has been solved variationally in Jacobi coordinates for J=0 and 1. Ten vibrational states of A1 and nine of B2 symmetry are calculated to lie below the linear minima. The permutational splitting between the (000)+ and (000)− states in the linear 12C2 11B has been calculated to be 0.064 cm−1, in 12C13C11B this is 0.530 cm−1. Above the energy of the barrier to linearity there are large amplitude vibrations with triangular structure character. In the dense stack of such states vibrational modes of the linear structure are discernible, including their permutational splittings.