Characteristics of the heavy rare gas dimers (Ar2,Kr2,Xe2) have been studied by correlated ab initio calculations. All-electron CCSD(T) calculations were performed for Ar and Kr dimers, and calculations with relativistic effective core potentials were performed for Kr and Xe dimers. Extended basis sets (aug-cc-pVXZ, X=D, T, Q, 5, 6) were combined with bond functions (spd, spdfg). The use of bond functions significantly improves the basis set convergence. For the argon dimer, we have included also a CCSDT correction yielding a higher quality potential energy curve. This correction has been calculated using aug-cc-pVTZ+spd basis set. All possible sources of errors have been analyzed for the argon dimer [basis set saturation, correlation contributions going beyond CCSD(T) method, effect of core corrections and relativistic corrections]. In the case of the Ar dimer, the highest level of theory reproduces the semiempirical stabilization energy within 1.3 cm−1. To obtain even closer agreement with experiment it would be necessary to fully include quadruple and higher excitations as well as to account properly for the core corrections with yet unpublished core oriented basis sets. Further improvement of one electron basis set will not lead to a better agreement with experiment. In the case of the other two dimers, the agreement between theory and experiment is also acceptable but not quantitative as in the case of the Ar dimer. Apparently, current calculations are close to the basis set limit and better agreement can only be obtained by proper covering of contributions mentioned for the argon dimer. The newly developed ECP oriented aug-cc-pVXZ basis set is very effective and can be recommended for high level calculations of molecular clusters containing heavier rare gas elements. The fast DZ/TZ extrapolation technique has been extended so that the use of empirical parameters can be avoided. Results obtained by extrapolations with medium size basis sets are surprisingly close to the most accurate ones. Further, the MP2–CCSD(T) difference was shown to be much less dependent on the size of the basis set than the energies themselves. These two conditions allow to construct the true stabilization energy of extended complexes as a sum of extrapolated complete basis set limit of MP2 stabilization energy and [MP2–CCSD(T)] term determined in a smaller basis set. The ab initio pair intermolecular potential results have been fitted to suitably chosen analytical formulas, and tested on experimental data for the second virial coefficients, spectral characteristics, and scattering data. For argon, an excellent agreement between the theoretical and the experimental values has been found. In the case of krypton and xenon the agreement is not as good but still acceptable.

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A detailed dynamical study is presented for N2 +/He collisions running in the electronic ground state of the collision complex. Hybrid, quantum-classical dynamical calculations have been performed considering a broad range of collision energies (Ecoll=0.01-100eV) and various initial rotational-vibrational excitations of the N2 + ion. Both non-reactive and reactive (N2 + collision-induced dissociation) cross-sections have been calculated with the momentum-transfer approximation applied to the former ones. A thorough comparison with pseudo-experimental data obtained from mobility measurements reported in the literature via an inverse-method approach has been performed and the effect of the rotational alignment of the N2 + ion on calculated cross-sections has been assessed and analyzed.

A new theoretical approach is presented for the general treatment of nonadiabatic hybrid dynamics (mixing classical and quantum approach) and applied to the postionization of rare-gas trimers. There was an important disagreement between trajectory surface hopping (TSH) or mean field (MF) approaches and the experimental results; noteworthy, with the new method qualitative and almost quantitative agreement is found for the fragmentation ratios of ionic monomers and dimers. For the first time in the theory as in the experiment, the dimers prevail for argon while monomers strongly dominate for the heavier rare gases, krypton and xenon. A new compromise between MF and TSH approaches is proposed and the new method is found quite robust with results not too sensitive to various possible implementations.

Rovibrational spectra of Ar3 are computed for total angular momenta up to J=6 using row-orthonormal hyperspherical coordinates and an expansion of the wave function on hyperspherical harmonics. The sensitivity of the spectra to the two-body potential and to the three-body corrections is analyzed. First, the best available semiempirical pair potential (HFDID1) is compared with our recent ab initio two-body potential. The ab initio vibrational energies are typically 1-2 cm-1 higher than the semiempirical ones, which is related to the slightly larger dissociation energy of the semiempirical potential. Then, the Axilrod-Teller asymptotic expansion of the three-body correction is compared with our newly developed ab initio three-body potential. The difference is found smaller than 0.3 cm-1. In addition, we define approximate quantum numbers to describe the vibration and rotation of the system. The vibration is represented by a hyper-radial mode and a two-degree-of-freedom hyperangular mode, including a vibrational angular momentum defined in an Eckart frame. The rotation is described by the total angular momentum quantum number, its projection on the axis perpendicular to the molecular plane, and a hyperangular internal momentum quantum number, related to the vibrational angular momentum by a transformation between Eckart and principal-axes-of-inertia frames. These quantum numbers provide a qualitative understanding of the spectra and, in particular, of the impact of the nuclear permutational symmetry of the system (bosonic with zero nuclear spin). Rotational constants are extracted from the spectra and are shown to be accurate only for the ground hyperangular mode.