Luís P. Viegas

Accurate ab initio based multisheeted double many-body expansion potential energy surface for the three lowest electronic singlet states of H3+.

The authors present diabatic and adiabatic potential energy surfaces for the three lowest electronic singlet states of H3+. The modeling of the surfaces is based on the multi-sheeted double many-body expansion method which consists of dressing the various matrix elements of the diatomics-in-molecules potential matrix with three-body terms. The avoided crossing between the two lowest states and the conical intersection between the second and the third state are accurately represented by construction.

Accurate double many-body expansion potential energy surface for triplet H3+. II. The upper adiabatic sheet (2 3A′)

We report on a global potential energy hypersurface for the upper sheet of the lowest triplet state of H3+. The analytic representation is based on the double many-body expansion theory. The ab initio data points, calculated with a large cc-pV5Z basis, are represented with a root mean square deviation of only 5.54 cm(-1) in the energy region below the H(+)+2H(2S) dissociation threshold. The quasi-bound vibronic states supported by this surface have also been calculated.

HO2 + O3 Reaction: Ab Initio Study and Implications in Atmospheric Chemistry.

We report a theoretical investigation on the reaction between ozone and the hydroperoxyl radical, which is part of the ozone depletion cycle. This reaction represents a great challenge to the state of the art ab initio methods, while its mechanism remains unclear to both experimentalists and theoreticians. In this work we calculated the relative energies of the stationary points along the reaction coordinate of the oxygen- and hydrogen-abstraction mechanisms using different levels of theory and extrapolating some of the results to the complete one-electron basis set limit. Oxygen abstraction is shown to be preceded by formation of hydrogen-bonded complexes, while hydrogen abstraction shows a lower energy barrier than oxygen abstraction. Both mechanisms lead to formation of HO3 + O2 in a very troublesome region of the potential-energy surface that is not correctly described by single-reference methods. The implications of the results on reaction dynamics are discussed.

Cone states of tri-hydrogen isotopomers and criterion for the geometric phase effect

Abstract We report calculations of the vibrational spectrum of μ H 2 /HD/DT as well as D 3 in their first-excited electronic state, both with the inclusion of the geometric phase effect and without including it. The results show that, especially for μ HD and μ DT, it plays a minor role for the first 30 vibrational levels of such systems. This can be rationalized from the tilting of the C 3 -axis due to mass effects. A simple phenomenological criterion for the relative role of the GP effect in tri-hydrogen isotopomers has been suggested.

Coupled‐cluster reaction barriers of HO2+H2O+O3: An application of the coupled‐cluster//Kohn–Sham density functional theory model chemistry

In this work, we report a theoretical investigation concerning the use of the popular coupled‐cluster//Kohn‐Sham density functional theory (CC//KS‐DFT) model chemistry, here applied to study the entrance channel of the HO2+H2O+O3 reaction, namely by comparing CC//KS‐DFT calculations with KS‐DFT, MRPT2//CASSCF, and CC//CASSCF results from our previous investigations. This was done by performing single point energy calculations employing several coupled cluster methods and using KS‐DFT geometries optimized with six different functionals, while conducting a detailed analysis of the barrier heights and topological features of the curves and surfaces here obtained. The quality of this model chemistry is critically discussed in the context of the title reaction and also in a wider context.

Role of (H2O)(n) (n = 2-3) Clusters on the HO2 + O3 Reaction: A Theoretical Study.

We report a theoretical investigation on the role of the water dimer and trimer in the reaction between the hydroperoxyl radical and ozone. This study is part of an ongoing series of research endeavors that intend to deliver a comprehensive understanding on the role of water on this reaction. Due to the complexity of the potential energy surface, and to be able to make comparisons with our previous works, our calculations have employed model chemistries based on the Kohn-Sham DFT formalism. It is found that the calculated reaction paths share a common scheme, not only in the context of this work, but also in consideration of our previous studies. Also, oxygen-abstraction barriers are always submerged, with the relative energy between the hydrogen- and oxygen-abstraction saddle-points increasing with the number of water molecules, which maintain an apparent spectator role. Finally, we report novel HO2···(H2O)3 and HO3···(H2O)n complexes originating from our reaction schemes.

Ro-vibrational states of triplet H3+ (a3Σu+): The lowest 19 bands

Abstract We have performed extensive calculations of the ro-vibrational states of triplet H 3 + , using the method of hyperspherical harmonics and our recently reported double many-body expansion potential energy surface. The rotational term values of the lowest 19 states are presented here for a total angular momentum of J ⩽10.