<div>Dear Stefano de Gironcoli: </div>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">>because they are obtained by a FFT on a discrete regular grid of q vectors.<br>>look (google or numerical recipes) for spectral analysis by Fourier<br>
>transform.<br>>An understanding of aliasing errors in FFT may also be useful.<br>>There is a relationship between the density of q-point grid and the<br>>extent to which interatomic force constants are accurately described in<br>
>real space... or rather the opposite ... between the extent of IFC in<br>>real space and the density of q-point grid needed to accurately describe<br>>them.</blockquote>
<div>Thanks again for your patient explanation.</div>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid"><br>>think of one spring connecting 2 balls.. E=1/2 K (x1-x2)^2<br>>what is the force between one atom and itself ?</blockquote>
<div>Sorry, I can not grasp what want tell me:(</div>
<div>Could you explain it more directly?</div>
<div><br>____________________________________<br>Hui Wang<br>School of physics, Nankai University, Tianjin, China </div>