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--- tutorials:paw1 [2016/04/24 13:09] – ↷ Page moved from howto:paw1 to tutorials:paw1 Yann Pouillon
+++ tutorials:paw1 [2016/04/24 13:09] (current) – ↷ Links adapted because of a move operation Yann Pouillon
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-==== [[| ]]****\\  3. The convergence on the double grid FFT cut-off ([[../input_variables/varpaw.html#pawecutdg|pawecutdg]]) **** ====
+==== [[howto:start| ]]****\\  3. The convergence on the double grid FFT cut-off ([[../input_variables/varpaw.html#pawecutdg|pawecutdg]]) **** ====
 In a// norm-conserving pseudopotential// calculation, the (plane wave) density grid is (at least) twice bigger than the wavefunctions grid, in each direction. In a //PAW// calculation, the (plane wave) density grid is tunable thanks to the input variable [[../input_variables/varpaw.html#pawecutdg|pawecutdg]] (PAW: ECUT for Double Grid). This is needed because of the mapping of  objects (densities, potentials) located in the augmentation regions (PAW spheres) onto the global FFT grid.\\  The number of points of the Fourier grid located in the spheres must be high enough to preserve the accuracy. It is determined from the cut-off energy  [[../input_variables/varpaw.html#pawecutdg|pawecutdg]]. An alternative is to use directly the input variable [[../input_variables/varpaw.html#ngfftdg|ngfftdg]]. One of the most sensitive objects affected by this "grid transfer" is the compensation charge density; its integral over the augmentation regions (on spherical grids) must cancel with its integral over the whole simulation cell (on the FFT grid).