You may also want to look at the full parameter list of all the buttons and sliders, or at one of the general help files, Sliders, or Graphs.
WRITING A DISLOCATION:
"burgers" allows you to add dislocations easily. To add a dislocation, click and drag (in any direction) with the right-hand mouse button (control--mouse button on the Mac). An arrow is drawn as the mouse is dragged. When you release the mouse button, a dislocation is created that is perpendicular to the arrow and runs through its tail. The width of the initial configuration of the dislocation is inversely proportional to the length of the arrow: a long arrow makes a narrow dislocation.
To create the dislocation, all the atoms on the "arrowed" side of the dislocation line are moved one lattice constant in the positive x-direction. To create a dislocation of opposite sign, hold down the shift key while dragging the mouse, or drag the arrow in the opposite direction.
WRITING A DEFECT:
"burgers" also allows you to specify the positions of impurities and vacancies arbitrarily. To do this, click on the atom that you want to configure. Each click will advance the atom through the list of possibilities: ...--impurity (yellow)--vacancy (clear: you may see a red atom below)--atom (blue)--impurity-- ... . Because the vacancies are represented as clear, with only a black outline, it is necessary to click on the black outline of the vacancy for "burgers" to notice your request.
The "impurity size" gives the equilibrium lattice constant of a lattice made up entirely of impurity atoms. The spring constant of the impurity is the same as for the host.
If a vacancy is present, the atoms on either side are coupled directly with one another with equilibrium distance c and the same force constants as the normal blue atoms. If a pair of atoms is separated by more than one vacancy, there is no coupling.
KEY EQUIVALENTS:
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Because the resolution of the calculation is much higher than that of the display, the system may appear static when, in fact, the atom velocities are increasing exponentially, but not yet with an observable magnitude. The <delta x> value gives the average x-component of the atomic displacements in the last calculational step. A brief look at this readout after a parameter change will quickly show whether the system is approaching a new equilibrium configuration or going unstable as the system surmounts an energy barrier impeding the motion.
STABILITY
The numerical algorithm becomes unstable for extreme values of some parameters. Stability may be improved by reducing the stepsize fraction in the configure window. Increasing the value of stepsize fraction speeds the program but may lead to instability.