Boundary 1.1

Boundary 1.1 - Boundary Conditions The length-scale of MD...

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University of Virginia, MSE 4270/6270: Introduction to Atomistic Simulations, Leonid Zhigilei Boundary Conditions The length-scale of MD is limited – a large fraction of the atoms is on the surface or “feel” the presence of the surface. How to reproduce interaction of atoms in the MD computational cell with the surrounding material? MD 1. Free boundaries (or no boundaries). This works for a molecule, a cluster or an aerosol particle in vacuum. Free boundary condition can be also appropriate for ultrafast processes when the effect of boundaries is not important due to the short time-scale of the involved processes, e.g. fast ion/atom bombardment, etc. keV particle bombardment, by Barbara Garrison http://galilei.chem.psu.edu/Research_bmb.html Examples of free boundary conditions in MD: Ultrafast process of sputtering Free cluster MD 2. Rigid boundaries (atoms at the boundaries are fixed). In most cases the rigid boundaries are unphysical and can introduce artifacts into the simulation results. Sometimes used in combination with other conditions (stochastic and periodic conditions, as discussed below).
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University of Virginia, MSE 4270/6270: Introduction to Atomistic Simulations, Leonid Zhigilei MD Boundary Conditions MD Large external system 3. Periodic boundary condition (eliminates surfaces – the most popular choice of boundary conditions). This boundary conditions are used to simulate processes in a small part of a large system. MD MD MD MD MD MD MD MD All atoms in the computational cell (green box) are replicated throughout the space to form an infinite lattice. Than is, if atoms in the computational cell have positions r i , the periodic boundary condition also produces mirror images of the atoms at positions defined as c n b m a l r r i image
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Boundary 1.1 - Boundary Conditions The length-scale of MD...

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