The solvation of the Na(+) ion in helium clusters has been studied theoretically using optimization methods. A many-body empirical potential was developed to account for Na(+)-He and polarization interactions, and the most stable structures of Na(+)He(n) clusters were determined using the basin-hopping method. Vibrational delocalization was accounted for using zero-point energy corrections at the harmonic or anharmonic levels, the latter being evaluated from quantum Monte Carlo simulations for spinless particles. From the static perspective, many-body effects are found to play a minor role, and the structures obtained reflect homogeneous covering up to n = 10, followed by polyicosahedral packing above this size, the cluster obtained at n = 12 appearing particularly stable. The cationic impurity binds the closest helium atoms sufficiently to negate vibrational delocalization at small sizes. However, this snowball effect is obliterated earlier than shell completion, the nuclear wavefunctions of (4)He(n)Na(+) with n = 5-7, and n > 10 already exhibiting multiple inherent structures. The decrease in the snowball size due to many-body effects is consistent with recent mass spectrometry measurements.

The potential energy curves, vibrational energy levels, spectroscopic constants, and dipole moment curves for the ground and excited states of BaXe and its ion Ba(+)Xe molecules are calculated with an ab initio method using pseudopotential techniques and core polarization potentials. The molecules are treated as two (BaXe) or one (Ba(+)Xe) active electrons systems taking benefit of the zero pseudopotential approach for Xe. The vibrational levels and their energy spacing have been also determined for Σ(+), Π, and Δ states. The permanent and transition dipole moment curves are investigated for the (1,3)Σ(+) states of the BaXe neutral molecule and (2)Σ(+) states of the Ba(+)Xe ion. The analysis of these numerous results shows interesting behavior in potential energy curves imprinted by the strong repulsive interactions between electron and Xe and also indicates an intense transition dipole moment for both Ba(+)Xe and BaXe.

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The structures and relative stabilities of mixed Ba(2+)Xe(n) (n = 1-39, 54) clusters have been theoretically studied using basin-hopping global optimization. Analytical potential energy surfaces were constructed from ab initio or experimental data, assuming either purely additive interactions or including many-body polarization effects and the mutual contribution of self-consistent induced dipoles. For both models the stable structures are characterized by the barium cation being coated by a shell of xenon atoms, as expected from simple energetic arguments. Icosahedral packing is dominantly found, the exceptional stability of the icosahedral motif at n = 12 being further manifested at the size n = 32 where the basic icosahedron is surrounded by a dodecahedral cage, and at n = 54 where the transition to multilayer Mackay icosahedra has occurred. Interactions between induced dipoles generally tend to decrease the Xe-Xe binding, leading to different solvation patterns at small sizes but also favoring polyicosahedral growth. Besides attenuating relative energetic stability, many-body effects affect the structures by expanding the clusters by a few percents and allowing them to deform more.

The spectroscopic properties of the fundamental and several excited states of Sr + Ar and SrAr, Van der Waals systems are investigated by employing an ab initio method in a pseudo-potential approach. The potential energy curves and the spectroscopic parameters are displayed for the 1-10 2 Σ + , 1-6 2 Π and 1-3 2 Δ electronic states of the Sr + Ar molecule and for the 1-6 1 Σ + , 1-4 3 Σ + , 1-3 1,3 Π and 1-3 1,3 Δ states of the neutral molecule SrAr. In addition, from these curves, the vibrational levels and their energy spacing are deduced for Σ + , Π and Δ symmetries. The spectra of the permanent and transition dipole moments are studied for the 1,3 Σ + states of SrAr, which are considered to be two-electron systems and 2 Σ + states of the single electron Sr + Ar ion. The spectroscopic parameters obtained for each molecular system are compared with previous theoretical and experimental works. A significant correlation revealed the accuracy of our results.