Solvation of potassium cation in helium clusters: Density functional theory versus pairwise method.

Microsolvation of a cation in helium quantum solvent is an attractive phenomenon leading generally to the formation of a strongly packed structure known as 'Snowball' feature. Here, the lowest energy structures and the relative stability of the solvated potassium cation K in helium clusters KHe up to the size n = 20 are investigated employing Density Functional Theory (DFT) and pairwise methods. The DFT calculations showed that M05-2X/6-311++G (3df, 2p) level of theory can reproduce properly the experimental data of KHe diatomic potential, whereas, in the pairwise method, the Basin-Hopping Monte Carlo (BHMC) algorithm was applied for the global optimization. The remarkable differences in the lowest energy structures computed in the frame of both methods are shown for KHe and KHe clusters. The BHMC optimization converged to an icosahedral geometry for n = 12, corresponding to the highest value of the binding energy per atom. For both methods, we have concluded that the first solvation shell is completed at the size n = 15, despite the maximum packing structure obtained at n = 17. Finally, the stability of the potassium doped helium cluster is discussed based on the Density Of States (DOS) curves.