Dear Stefano de Gironcoli:
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<div>First, thanks for you answer:)</div>
<div>I draw a lot of inspiration from your reply:P</div>
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<div>But I am still confused about one thing.</div>
<div>" 4 4 4<br> 1 1 1 1<br> 1 1 1 2.24396850738E-01"<br><font color="#000000">I think <font color="#006600">F_{1,1}^{x,x}(R(1,1,1))</font> is force constant between the <font color="#006600">1th atom </font>and<font color="#006600"> itself</font>.</font></div>
<div><font color="#550055"><font color="#000000">What the meaning of</font> "<font color="#550055"><font color="#009900">force constant between one atom and its self</font>"?</font></font></div>
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<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">>Notice also that the force constant grid is written as these were<br>>periodic ...<br>>so in a 4x4x4 grid R(1,1,1) = 0, R(2,1,1,) = a1, and R(4,1,1)=-a1</blockquote>
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<div>You said that "force constants are periodic".</div>
<div> I think you are right.But I do not know why it is periodic.</div>
<div><br>____________________________________<br>Hui Wang<br>School of physics, Nankai University, Tianjin, China </div>