Martina Bittererová

A theoretical study of the azide (N3) doublet states. A new route to tetraazatetrahedrane (N4): N+N3→N4

The potential energy surfaces for the low-lying doublet states of the azide radical (N-3) have been computed at the complete active space self-consistent field (CASSCF) level with the CAS(15,12) ac ...

Spectroscopy of ozone at the dissociation threshold: Quantum calculations of bound and resonance states on a new global potential energy surface

The spectroscopy of O3() near the O(3P) + O2(3Σg−) threshold is investigated by means of quantum mechanical dynamics calculations (filter diagonalization with absorbing potential) on a global potential energy surface for the ground electronic state, 1A′(1A1), obtained from multi-reference configuration interaction calculations. Most of the highly excited bound states and even a substantial number of the resonance states can be clearly assigned. As a result, the resonance widths show a pronounced state specificity with the longest lifetime being of the order of 1 ns—three orders of magnitude below the prediction of transition state theory.

On the S1 → S0 internal conversion in the photodissociation of HNCO

Abstract We discuss a possible pathway for the S 0 /S 1 internal conversion in the photodissociation of HNCO in the first absorption band. For this purpose, two-dimensional potential energy surfaces for the lowest two singlet states are calculated using the multi-reference configuration-interaction ab initio method; the NCO bending angle and the out-of-plane torsional angle are varied. According to our calculations, the transition from the S 1 to the S 0 state is likely to occur near a conical intersection at small NCO angles, which can be reached by large-amplitude NCO bending motion.

Quasiclassical trajectory study of H + O2 → OH + O at temperatures from 500 to 2000 K

Abstract Classical trajectory calculations for the title reaction were carried out on the new extended LEPS potential energy surface. To compare the thermalized rate constants k(T) with the results of direct experimental measurements, reaction cross sections were calculated for specific values of vibrational and rotational quantum numbers and relative translational energies. Our results predict a smaller increase of k(T) than the previous studies. The temperature dependence of dynamic characteristics for the reactive and non-reactive collisions were also examined. Finally, the influence of rotational, vibrational and translational enhancement on the characteristic features of reactive trajectories was investigated.

On the S1→S0 internal conversion in the photodissociation of HNCO: the role of the NC stretch as a promoting mode

Abstract A possible pathway for the S 1 /S 0 internal conversion in the photodissociation of HNCO in the first absorption band is discussed on the basis of electronic structure calculations for the two relevant potential energy surfaces. The ab initio calculations are performed at two levels of theory: multi-reference configuration interaction (MRCI) and complete-active space second-order perturbation theory (CASPT2). The NCO bending angle and the NC stretch coordinate are varied. It is concluded that the combined motion of both modes is most efficient in promoting the transition from S 1 to S 0 , which is in qualitative accordance with the recent experimental findings.

The barrier topography of the H + F2 potential energy surface

The potential energy surfaces (PESs) for the electronic ground state and the three lowest excited states of the H + F2 reaction have been studied by ab initio multireference singles and doubles configuration interaction (MRCI-SD) calculations. Our main focus is laid on the description of the angular dependence of the barrier region on the ground state surface. In general, except for the cases of increased symmetry, the barrier arises from avoided-crossing interactions of the two lowest 2A′ states. In the collinear case it is the interaction between the two lowest Σ states which is responsible for the barrier. Due to the weak interaction of these well-separated and not even neighboring states the barrier is relatively low. For broadside approach of H onto F2 in C2v symmetry the avoided crossing transforms into a symmetry induced conical intersection between the lowest 2A1 and 2B1 states. With our largest calculation we obtain a height of 0.130 eV (0.078 eV including the Davidson correction) for the collinear barrier, and of 1.12 eV (0.86 eV) for the broadside barrier at the conical intersection. We also consider semiempirical PESs frequently used for simulations of the H + F2 reaction. In comparison to our MRCI-SD results, a diatomics-in-molecules PES shows a qualitatively different angular dependence of the barrier in the broadside range. The implications of the barrier topography for the interpretation of experimental findings are discussed.

Systematic generation of relativistic gaussian basis sets

A method for the Gaussian basis set generation for molecular relativistic Dirac-Fock calculations is proposed. The basis set exponents are obtained in the process of stochastic optimization (a hybrid of simplex and simulated annealing optimization techniques has been employed) of a functional defined as the sum of squares of differences between the numerical relativistic atomic wave functions and the wave functions obtained using the Gaussian function expansion. After this pre-optimization step the exponents are refined by ordinary gradient energy-functional based procedure. The present method seems to be very effective and robust. As an example the optimized basis sets of atoms from H (Z=1) to Ar (Z=18) are presented. Results of the Dirac-Fock calculations for all atoms under study are presented and compared with the numerical Dirac-Fock results and results obtained using the Gaussian basis sets according to Okada et al.: J. Chem. Phys.93 (1990) 5013.

A comparative study of different approximations in the coupled-cluster method

Abstract The non-degenerate version of the coupled-cluster method has been used to examine the impact of different levels of excitations (up to triply-excited states) on calculated spectroscopic constants of diatomic hydrides. The results presented show that for obtaining reasonable data inclusion of the T 3 clusters (at least in a non-iterative fashion) is very important.