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5.6 About nr1b, nr2b, nr3b

ecutrho defines the resolution on the real space FFT mesh (as expressed by nr1, nr2 and nr3, that the code left on its own sets automatically). In the ultrasoft case we refer to this mesh as the "hard" mesh, since it is denser than the smooth mesh that is needed to represent the square of the non-norm-conserving wavefunctions.

On this "hard", fine-spaced mesh, you need to determine the size of the cube that will encompass the largest of the augmentation charges - this is what nr1b, nr2b, nr3b are.

So, nr1b is independent of the system size, but dependent on the size of the augmentation charge (that doesn't vary that much) and on the real-space resolution needed by augmentation charges (rule of thumb: ecutrho is between 6 and 12 times ecutwfc).

In practice, nr1b et al. are often in the region of 20-24-28; testing seems again a necessity (unless the code started automagically to estimate these).

The core charge is in principle finite only at the core region (as defined by rcut ) and vanishes out side the core. Numerically the charge is represented in a Fourier series which may give rise to small charge oscillations outside the core and even to negative charge density, but only if the cut-off is too low. Having these small boxes removes the charge oscillations problem (at least outside the box) and also offers some numerical advantages in going to higher cut-offs.

The small boxes should be set as small as possible, but large enough to contain the core of the largest element in your system. The formula for determining the box size is quite simple:

nr1b = (2 x rcut)/Lx x nr1

where rcut is the cut-off radius for the largest element and Lx is the physical length of your box along the x axis. You have to round your result to the nearest larger integer." (info by Nicola Marzari)


next up previous contents
Next: 6 Performance issues (PWscf) Up: 5 Using CP Previous: 5.5 Conjugate Gradient Contents
Paolo Giannozzi 2009-10-01