<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">Dear Hanghui: I think this is just because of periodic boundary conditions. Thinking in 1D, as always, helps. Suppose you have a cell of size L. You place one atom, say A, at the origin (x=0), and another one, say B, at x=2/3 L. What will be the distance between A and B? Not 2/3 L, but L/3!<div><br class="webkit-block-placeholder"></div><div>Hope this helps</div><div>Stefano Baroni</div><div><br><div><div>On Nov 9, 2007, at 4:02 AM, alan chen wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>To whom it may concern,</div> <div> I am trying the CONSTRAINTS card for ionic dynamics. One of the 'constr_type' is 'distance'. However, if the distance of the two atoms I want to fix is larger than 'dmax' (dmax is half the shortest edge of the cell), the code crashes. </div> <div> I would like to ask why we can not fix two atoms whose distance is NOT larger than 'dmax'? Does the algorithm not work well if the distance of two atoms to be fixed is too large?</div> <div> Thank you very much.</div> <div> Hanghui</div> <div> Department of Physics,</div> <div> Yale University</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">_______________________________________________</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">Pw_forum mailing list</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><a href="http://www.democritos.it/mailman/listinfo/pw_forum">http://www.democritos.it/mailman/listinfo/pw_forum</a></div> </blockquote></div><br><div> <span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; 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