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Small (4)He clusters doped with a single atomic impurity, (1)H, (2)H, and (3)H, have been studied via a quantum Monte Carlo approach with the intent of establishing their binding behavior in nanoscopic clusters. Our calculations find that the only trimer (x)H (He)(2), which exhibits a bound state, is that with the tritium dopant (x = 3), in agreement with previous calculations using hyperspherical coordinates in the adiabatic approximation. The lightest dopant (1)H is seen not to stabilize the small helium clusters, while (2)H and (3)H are weakly bound to this solvent: our computed exchange energies and probability distribution functions reveal the "heliophobic" nature of hydrogen, leading thus to a further confirmation, in the realm of nanoscopic-size systems like the helium droplets, of the well-known nonmixing and nonsolvating features of hydrogen in macroscopic liquid bulk helium.

The structure and energetics of (3,4)HeCs(2)((3)Sigma(u)) molecules are analyzed from first principles. Fixing the cesium dimer at its equilibrium distance, the electronic structure was determined through ab initio methods at the CCSD(T) level of theory using a large basis set to compute the interaction energies. At the T-shaped geometry, there is a shallow well with a depth of approximately 2 cm(-1) placed at R approximately 6.75 A, R being the distance from the center of mass of Cs(2) to He. That depth gradually decreases to approximately 0.75 cm(-1), while R increases to about 11.5 A at linear arrangements. A simple model of adding atom-atom Lennard-Jones potentials with well-depth and equilibrium distance parameters depending on the angular orientation was found to accurately reproduce the ab initio points. Using this analytical form, variational calculations at zero total angular momentum are performed, predicting a single bound level at approximately -0.106 (approximately -0.042) cm(-1) for the boson (fermion) species. Further calculations using Quantum Monte Carlo methods are carried out and found to be in good agreement with the variational ones. On the basis of the present results, such analytical expression could in turn be used to describe the structure and binding of larger complexes and therefore opens the possibility to further studies involving such aggregates.

Ab initio computed interaction forces are employed in order to describe the microsolvation of the A$_2^+(^2\Sigma)$ (A=Li,Na,K) molecular ion in $^4$He clusters of small variable size. The minimum energy structures are obtained by performing energy minimization based on a genetic algorithm approach. The symmetry features of the collocation of solvent adatoms around the dimeric cation are analyzed in detail, showing that the selective growth of small clusters around the two sides of the ion during the solvation process is a feature common to all three dopants.