Determination of the Stable Structures and Phase Transitions of Adamantane Nanoclusters by Molecular Dynamics Simulation

Adamantane (C6H12) is the smallest possible of diamondoids with a highly symmetric and point group symmetry, Td. A cage like structure is formed with six CH2 and four CH groups giving rise to a molecular structure with four cyclohexane rings in chair form. Due to their six or more linking groups, they have found major applications as templates and as molecular building blocks in nanotechnology, drug delivery, drug targeting [1-3]. This report is aimed at studying the stable structure and phase transition behaviors of adamantane nanoclusters for further understanding of their structures and possibly building molecular devices with them. Computational method: Applying ab initio calculation and molecular dynamics simulation method, we have been calculated and predicting the stable structure and phase transition of adamantane nanoclusters. We have chosen a large number of nanocluster systems consisting of 5, 10, 13, 15, 20, 25, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140 and 147 molecules of adamantane. We first performed (DFT) calculation to optimize initial geometric structure of adamantane. In order to find the equilibrium configurations and phase transition of the mentioned collections of the 5-147 molecules at every given temperature we performed MD simulation for these systems until they wereequilibrated, and to gain phase transition of these nanoclusters we used the simulatedannealing procedure. Then their phase transitions have been obtained via ploting ofdiagram of the lindemann index versus temperature. Results and discussion: As shown in Fig.1 (a) Increasing the number of particles does not increase the phase transition temperature regularly, and the higher phase transition temperatures are related to the symmetric structures of icosahedral belong to 13, 55,135 and 147 nanoclusters. Because the structures proceed by increasing the particles toward the formation of an icosahedral stable structure, hence some of the structures relative to their close family is more unstable and less phase transition point. In fact, this process is similar to the increase of electrons in a period of the periodic table, which evolve until the formation of the closed shell of noble gaseous. Fig.1 (b) and Fig.1(c) show the icosahedral stable structure for 13 and 55 nanoclusters.