<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "> <br><div><div>On 3 Feb 2008, at 19:04, xiaoliang wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">I wonder how cp2k calculate the number of grid points? Because If I</font></p> <p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">change the size of reference cell, the number of grid points is</font></p> <p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">changed as well, I think. Then do we need to<span class="Apple-converted-space"> </span>make energy vs reference</font></p> <p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">cell size plot to get energy convergece, like the test for energy vs</font></p> <p style="margin: 0.0px 0.0px 0.0px 0.0px"><font face="Helvetica" size="3" style="font: 12.0px Helvetica">cutoff ?</font></p> </blockquote></div><br><div>In principle yes.. but you would discover that to get "full convergence" you should reach extremely high cutoff</div><div>(unaffordable for standard calculations).</div><div>However, the nice thing is that the absolute energy is just shifted and the shape of the potential energy does not change </div><div>sensibly. This makes feasible to use a relatively low cutoff (of course too small creates problems as well..).</div><div>"Generally speaking" a cutoff of around 300 Ry is most of the time enough.. of course the real good number depends on the</div><div>nature of your basis set, pseudo, XC.</div><div><br class="webkit-block-placeholder"></div><div>Ciao,</div><div>Teo</div></body></html>