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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.
  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.

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