Energetics, Thermal And Structural Properties Of Hafnium Clusters Via Molecular Dynamics Simulation
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Date
2016-09
Authors
Ng, Wei Chun
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Abstract
The melting behavior of hafnium clusters (of sizes 2<𝑛<99) are studied via molecular dynamics (MD) simulation. The interaction between the hafnium atoms is described by Charged-Optimized Many-Body (COMB) potential. The same COMB potential is used with a global optimization algorithm called PTMBHGA to generate the input ground state structures for MD processes. These assumed ground state structures are verified as compared to the literature and first-principles calculation, which further confirm the dependability of COMB potential within the MD processes. Conventionally, the energy parameters are used to evaluate the properties of a cluster. This thesis implements the use of geometry of the clusters in additional to the caloric profile to evaluate the dynamics during cluster melting. Global similarity index, a purpose-designed algorithm to quantify the degree of similarity between two clusters is formulated to achieve this objective. It is derived based on the chemical similarity of molecule and fulfil similar property principle. The heating MD process is carried out either using direct heating or prolonged simulated annealing. Melting point is identified by caloric curve, heat capacity curve and global similarity index. The melting point of hafnium cluster changes with the size of the cluster, 𝑛. In addition to that, the melting transition happens across a range of temperature, starting with a pre-melting stage at temperature 𝑇𝑝𝑟𝑒 to total melting at a higher temperature 𝑇𝑚. All the three methods agree with each other for the range of melting temperature for hafnium
clusters. However, it is found that global similarity index is much more superior, as it also traces the melting mechanism of hafnium clusters.
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The use of geometry of the clusters in additional to the caloric profile , to evaluate the dynamics during cluster melting.