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The stability of four fermionic particles with unit charge, of which two are positively and two negatively charged, is discussed. Except for using the simplest approximation of a single Gaussian orbital per particle, the problem is exactly solved variationally by varying the masses to simulate molecular di-hydrogen, mono-muonated di-hydrogen, and di-muonated di-hydrogen. We illustrate the celebrated Born-Oppenheimer approximation 2 years after the occasion of its 90th anniversary. It is suggested that this method is valid only for di-hydrogen. Key words: muonated hydrogen, Born-Oppenheimer approximation, [micro]-mesonic molecules, interatomic forces, stability of fermionic particles. Nous examinons la stabilite de particules faites de quatre fermions portantune charge unitaire, deux positives et deux negatives. Excepte pour l'approximation d'une simple orbitale gaussienne par particule, le probleme est solutionne exactement par methode variationnelle et en variant les masses pour simuler les complexes moleculaires de dihydrogene et de dihydrogene a un muon et a deux muons. Ce calcul sert a illustrer la celebre approximation de Born-Oppenheimer, 2 ans apres son [90.sup.e] anniversaire. Nous concluons que cette approximation n'est valide que pour le dihydrogene. [Traduit par la Redaction] Mots-cles : hydrogene muonique, approximation de Born-Oppenheimer, molecule a muon [micro], force interatomique, stabilite de particules faites de fermions., 1. Introduction The understanding of physics and chemistry rests on a hierarchy of approximations, with the one carrying the names of Born and Oppenheimer [1] (BO) being most celebrated. At [...]

A single-valued double many-body expansion potential energy surface is obtained for the ground electronic state of the hydrogen cyanide molecule (HCN) from multiproperty fit to ab initio energies and spectroscopic rovibrational data. This includes 106 rovibrational levels and 2313 discrete points and seven zero first-derivatives that are reproduced at three stationary points.

A single-valued double many-body expansion potential energy surface is reported for ground-state HN2 by fitting accurate ab initio energies that are suitably corrected by scaling for the H-N2 and N-NH regions. Examination of the topographical features of the novel global potential energy surface is done in detail.

The steady-state distribution of O(sub 2) and OH in the high atmosphere, from detailed trajectory conditions, which shows that the thermalization assumption are untenable at such environmental conditions is reported.

The quasi classical trajectory (QCT) study was done of the title reaction for several combinations of vibrational excitations of the reactant species. It was revealed that with the increasing vibrational energy of the HO2 molecules, an abstraction reaction occurred through prompt breaking and formation O-O bonds due to which the lifetime of the ozone species was short.

A quasiclassical trajectory (QCT) study of the molecular dissociation in the OH + O2 reaction for several combinations of rotational and vibrational quantum numbers is presented. The results show that OH dissociation and ozone formation via collisions of vibrationally excited OH and O2 molecules are comparable over the same range of collisional energies and that the title dissociative processes provide extra sources of ozone through a three-body recombination reaction with molecular oxygen.

A study of accurate single-valued double many-body expansion (DMBE) potential energy surface for the ground electronic state of HSO based on novel MR CISD ab initio energies suitably corrected for the complete one-electron basis-set/complete CI limit is presented. A n-body distributed polynomial approach, which implies using individual multinomial developments at the various stationary points is suggested.

Calculations are done on HCl, OCl and other states at the ab initio MRCl level, and modeled semiempirically using the extended Hartree-Fock approximate correlation energy method. In conclusion, vibrational levels of various states are calculated, and found to be in good agreement with existing spectroscopic Rydberg-Klein-Rees (RKR) data for the ground electronic states of the title diatomic molecules.

Research concerning evaluation of the internal partition function of diatomic and triatomic van der Waals molecules is presented. Simulation methods yield accurate results when compared with corresponding quantum mechanical results.

Research was conducted to examine the role of the chaperon mechanism through ArCN or ArH complex formation for the association reaction H + CN + Ar -> HCN + Ar. The objective was to investigate some assumptions with focus on the branch that involves the ArCN van der Waals molecule. Results demonstrate the significance of the chaperon mechanism only at low temperatures. Also, the energy-transfer mechanism was found to dominate for both high and moderate temperatures.

A detailed theoretical study of the branching reaction HO(v'=0) + O2(v') -> HO2 + O was performed by applying the quasiclassical trajectory technique and a single-valued double many-body expansion potential energy surface for the electronic ground state of HO3. Computations indicated that the reaction occurs for vibrational quantum numbers larger than v' = 13 over the whole range of translational energies. In most cases, no breaking of the HO bond occurs.

Three-dimensional quantum dynamics computations of the atmospheric reaction H + O3 -> HO + O2 are discussed. Computations were performed using single-path (SP) and multiple-path (MP) approaches of a novel elastic optimum angle adiabatic scheme. It is shown that the SP approach yielded lower cross sections compared to those provided by the QCT theory while the results of the MP approach were in good agreement with QCT values.

New quantum dynamics calculations of the rate constant for the atmospheric reaction H + O3 -> O2 + OH were performed. These were conducted under the assumption of an atom-to-atom reactive process scheme instead of on the basis of the global atom-molecule reactive scattering technique previously presented. Agreement with available classical trajectory results have been substantially enhanced even though the new theoretical results still continue to agree best with the more recent experimental data of Cline and Monkhouse and Greenblatt and Wiesenfeld.

The quasiclassical trajectory method was integrated with double many-body expansion (DMBE) potential energy surface for electronic ground-state HO3 to evaluate the environmental reaction O + HO2 yields OH + O2. Results support the reliability of the HO3(super2A) DMBE potential energy surface. In addition, the experimental DMBE conditions also shows oxygen atom abstraction as the only vehicle for the title reaction.