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FAQ: Why is my energy vs. volume plot jagged

This is a very common questions from people who start to do calculations with plane wave codes. There are two reasons why the energy vs. volume plot looks jagged:

  1. Basis set incompleteness. The basis set is discrete and incomplete, and when the volume changes, additional plane waves are added. That causes small discontinuous changes in the energy.
    Solutions:
  2. However the most probable reason for the jagged E(V) curve is another one: For PREC=High the FFT grids are chosen so that $ {\bf H} \vert \phi>$ is exactly evaluated. For PREC=Med the FFT grids are set to 3/4 of the value that is in principle required for an exact evaluation of $ {\bf H} \vert \phi>$. This introduces small errors, because when the volume changes the FFT grids do change discontinuously. In other words, at each volume a different FFT-grid is used, causing the energy to jump discontinuously.
    Solutions:

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Next: The most important parameters, Up: Volume vs. energy, volume Previous: Accurate bulk relaxations with   Contents   Index
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