Characterization of small pure and Ni-doped titanium clusters: Ab initio versus classical approaches

The optimized structure of pure and Ni-doped titanium clusters consisting of up to 15 atoms is studied using ab initio theoretical methods, based on the density-functional theory, and classical simulations as well. The simulations are performed using classical force fields for Ti–Ti and Ni–Ti interactions, which are parameterized to reproduce bulk properties of perfect Ti and NiTi crystalline structures. By comparing the results obtained within the two different approaches, we show that the classical force fields are also applicable for the description of significantly smaller systems, such as atomic clusters. This indicates also that the existing force fields describe adequately not only the macroscopic properties of the crystalline structures, but also point defects and local irregularities which may occur in a crystal. We perform also the ab initio analysis of electronic and magnetic properties of the clusters and show that the doping of titanium clusters by Ni atoms causes a significant charge transfer in a system and the change of the magnetic properties as well.