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Unformatted text preview: Molecular Dynamics Simulations 2011 Exercise 10: Suggested solutions Eero Holmstr¨ om November 23, 2011 1. First we relax the periodic cell (either 5 × 5 × 5 or 7 × 4 × 4) by running for 10 ps at 0 K using btctau=300.0 and bpctau=300.0 . Then, we open up the z-direction and run for 10 ps with btctau=300.0 and bpctau=0.0 . Here the pressure control is turned off, because it doesn’t work correctly if free surface is present. We take the potential energies of the final steps of the simulations and calculate the surface areas from the relaxed cell sizes. Finally we can calculate the surface energy as γ = E 2- E 1 2 A . (1) Results are summarized in Table 1. The experimental results given in the reference are either 1.79 or 1.83 J/m 2 for both surfaces, and the ab initio results are 2.17 and 1.83 J/m 2 for the (001) and (111) surfaces, respectively. The Morse results are clearly off these values. Even the order of the surface energies varies with the potential cut-off (5 ˚ A vs. 7 ˚ A). When the atoms interact only with their nearest neighbors (as with r c = 3 . ˚ A), the force between each atom pair is equal to zero. Opening up the surface then doesn’t cause any relaxation. However, when when we take into account more than just the nearest neighbor (as with...
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