helium, quantum Monte Carlo, rare gas clusters

The main objective of the present study is a detailed theoretical description of structural properties, stability, and energetics of ionized helium clusters. Our objects are interesting due to experiments performed in helium nanodroplets at very low temperatures and weak perturbations, taking benefits of helium superfluidity. For theoreticians, the field of ionized helium clusters is the great challenge, too. On the one hand, there is a failure of the semiempirical diatomics-in-molecules interaction model due to an insufficient inclusion of three-body forces; on the other hand, for these quantum systems of light nuclei it is not possible to use only stationary points on PES for exact calculations of ground state properties of these species. We can expect a strong quantum behavior for Hen+ as well as experimental magic numbers not fully compatible with simple considerations based on the analysis of stationary points only. The results obtained via DMC simulations have been compared with experiments.

Three-body contributions to the interaction energy of Ar3 have been calculated recently using the HF - CCSD(T) method and multiply augmented basis sets. Tests of new ab initio three-body potential for argon trimer are performed in this work. These tests consist in a comparison of several quantities we calculated with corresponding recent theoretical results and available experimental values. Recently published ab initio pair potential is used. The first part of the tests include data on rare-gas solids - a zero-temperature binding energy, lattice constant, and crystal structure. The second part of the tests is focused on the third virial coefficient - one of thermodynamic quantities which are influenced exclusively by two and three-body intermolecular interactions. In addition, the solid and gas phase properties considered are also calculated using semiempirical pair potential and simple Axilrod-Teller three-body term for comparison.