Dear Stefano de Gironcoli:
> >because they are obtained by a FFT on a discrete regular grid of q > vectors. > >look (google or numerical recipes) for spectral analysis by Fourier > >transform. > >An understanding of aliasing errors in FFT may also be useful. > >There is a relationship between the density of q-point grid and the > >extent to which interatomic force constants are accurately described in > >real space... or rather the opposite ... between the extent of IFC in > >real space and the density of q-point grid needed to accurately describe > >them. Thanks again for your patient explanation. > > >think of one spring connecting 2 balls.. E=1/2 K (x1-x2)^2 > >what is the force between one atom and itself ? Sorry, I can not grasp what want tell me:( Could you explain it more directly? ____________________________________ Hui Wang School of physics, Nankai University, Tianjin, China -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20090801/f525cf9e/attachment.htm