thanks for all your replies.i shall try it according to your advices.<br><br>
<div><span class="gmail_quote">2007/7/15, Stefano Baroni <<a href="mailto:baroni@sissa.it">baroni@sissa.it</a>>:</span>
<blockquote class="gmail_quote" style="PADDING-LEFT: 1ex; MARGIN: 0px 0px 0px 0.8ex; BORDER-LEFT: #ccc 1px solid">
<div style="WORD-WRAP: break-word">Hi All:
<div><br> </div>
<div>a small further comment to explicitly stress a point that has already been raised more or less implicitly.
<div><br> </div>
<div>One runs simulations not just for the sake of it, but to calculate properties, right? And properties are numbers, right? </div>
<div><br> </div>
<div>Well, the right questions to ask at the start of a simulation concern the influence of the input parameters on the *specific numbers* one wants to calculate (i.e. the *specific property* one wants to study). Of course, the energy is a very important property, and it is very correct to ask oneself what the influence of the input parameters on the accuracy of the calculated energy would be (not to mention its conservation, if one runs a MD simulation). It is not the only one, though. If you want to study the equilibrium geometry, for instance, it is the accuracy of bond lengths and bond angles that would matter, and the softer the system, the more difficult it will be to achieve a given target accuracy (this point has been discussed in this very thread, I think).
</div>
<div><br> </div>
<div>All in all, I would recommend that before asking what value a given parameter should be given, one should ask oneself "why should I care about this parameter?". A proper answer to the latter question will almost invariably also answer the first.
</div>
<div><br> </div>
<div>I understand that the usage of canned packages such as QE or vasp encourages a somewhat uncritical approach to the many, many, parameters needed to fine tune a well done simulation. This is one more reason, I think, to make an effort to ask sometimes general questions before specific ones.
</div>
<div><br> </div>
<div>Cheers - Stefano</div>
<div><br>
<div><span class="q"><br>
<div>
<div>On Jul 15, 2007, at 5:29 AM, Xunlei Ding wrote:</div><br>
<blockquote type="cite">
<div style="MARGIN: 0px">Hi,</div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MARGIN: 0px">In the vasp forum, one set the parameter like this:</div>
<div style="MARGIN: 0px">EDIFF: 1E-07</div>
<div style="MARGIN: 0px">EDIFFG: -0.0005</div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MARGIN: 0px">And in the reply it is said, "Anyways, your ediffg is very low. A strict force</div>
<div style="MARGIN: 0px">criteria is -0.001, and many people would use -0.01 or higher. If you look at</div>
<div style="MARGIN: 0px">the change in the energy as you go from a force of 0.01 eV/Ang to 0.001 eV/Ang,</div>
<div style="MARGIN: 0px">it is usually negligible."</div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MARGIN: 0px">You can check the difference of results when you set force_conv_thr as 1.d-3 and</div>
<div style="MARGIN: 0px">1.d-4. If the difference is negligible, why use too strict threshold?</div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MARGIN: 0px">Yours,</div>
<div style="MARGIN: 0px">Ding Xunlei</div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MIN-HEIGHT: 14px; MARGIN: 0px"><br> </div>
<div style="MARGIN: 0px">Quoting xu yuehua <<a onclick="return top.js.OpenExtLink(window,event,this)" href="mailto:njuxuyuehua@gmail.com" target="_blank">njuxuyuehua@gmail.com</a>>:</div></blockquote></div><br></span>
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<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">---</span></span></span></span></span></span></span></span></span></font></div>
<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">Stefano Baroni - SISSA</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">&</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">DEMOCRITOS National Simulation Center - Trieste</span></span></span></span></span></span></span></span>
</span></font></div>
<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)</span></span></span></span></span></span></span></span></span></font></div>
<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><br></span></font></div>
<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">Please, if possible, don't</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">send me MS Word or PowerPoint attachments</span></span></span></span></span></span></span></span></span>
</font></div>
<div style="MARGIN: 0px"><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">Why? See:</span></span></span></span></span></span></span></span></span></font><font size="3"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px">
<span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"><span style="FONT-SIZE: 12px"> </span></span>
</span></span></span></span></span></span></span></font><a onclick="return top.js.OpenExtLink(window,event,this)" href="http://www.gnu.org/philosophy/no-word-attachments.html" target="_blank"><font color="#0023e9" size="3">
<span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)">
<span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)"><span style="FONT-SIZE: 12px; COLOR: rgb(0,35,233)">http://www.gnu.org/philosophy/no-word-attachments.html
</span></span></span></span></span></span></span></span></span></font></a></div>
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</div><br><br clear="all"><br>-- <br>Xu Yuehua<br>physics Department of Nanjing university<br>China