<div>Dear Paolo Giannozzi:</div>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">>what is the meaning of "force constants"? ( hint: second derivative of<br>>the energy with respect to atomic displacements )</blockquote>

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<div>Thanks for your hint. I got it:)</div><br>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid"><br>____________________________________<br>Hui Wang<br>School of physics, Nankai University, Tianjin, China </blockquote>
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