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On 05/22/2011 05:57 AM, S. D. Wang wrote:
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cite="mid:BANLkTi=7Zyy2xB=r+SEsBdQfrG8Gjj-jfA@mail.gmail.com"
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<pre wrap="">Dear developers:
In scf.out file, it has 'G cutoff = 122.3770 ( 89195 G-vectors) FFT
grid: ( 24, 40,216)' , I want to kown the relation of cutoff and G-vectors.
How can I understand the 122.3770 and 89195</pre>
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G = b1*n1+b2*n2+b3*n3: where b1,b2,b3 are fundamental lattice
vectors of the reciprocal lattice.<br>
<br>
All G such that G^2 < Ecut_rho are included in the G vector
list. G cutoff is just Ecut_rho in unit of (2pi/a)^2 (or maybe the
sqrt of it). The number of G in the cutoff sphere is in your case
89195.<br>
<br>
the FFT grid dimensions are set in such a way that the sphere is
contained in it. FFT grid contains 24*40*216= 207360 which is about
twice the number of G in the list. It is the expected ratio as the
volume between a sphere and the cube containing it is pi/6<br>
<br>
stefano<br>
<br>
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