Dear Amin:<div><br><div class="gmail_quote">On Sat, May 28, 2011 at 2:26 AM, Amin Torabi <span dir="ltr"><<a href="mailto:amin.torabi@gmail.com">amin.torabi@gmail.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div dir="ltr"><div>Dear all,</div><div><br></div><div>I did a vc-relax calculation. Then, I used the cell_parameters and atomic_positions obtained at the end of vc-relax to do an scf calculation. I didn't change any other settings, like k-point or ecutoff. </div>
<div>However, the total energy, the total force and the total stress obtained from the scf calculation are different form the values obtained at the end of the vc-relax calculation. (The vc-relax values are lower.)</div>
<div>
Why is it so?</div></div></blockquote><div>You are right. </div><div>When you do vc-relax, the G points contained in wave-function does not change with the cell parameters(That means the energy cutoff changes when cell parameters change during vc-relax). When you do scf after vc-relax, the cell parameters of scf is different from that of vc-relax.</div>
<div>Therefore, the G points contained in wave-function of scf differs from that of vc-relax.</div><div>Usually, you should do the second vc-relax if the cell parameters change a lot. After the second vc-relax, the force/stress of scf will be close to that of second vc-relax.</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div dir="ltr"><div><br></div><div><br></div><div>Thanks is advance.</div><div><br></div><font color="#888888"><div>
Amin</div></font></div>
<br>_______________________________________________<br>
Pw_forum mailing list<br>
<a href="mailto:Pw_forum@pwscf.org">Pw_forum@pwscf.org</a><br>
<a href="http://www.democritos.it/mailman/listinfo/pw_forum" target="_blank">http://www.democritos.it/mailman/listinfo/pw_forum</a><br>
<br></blockquote></div><br><br clear="all"><br>-- <br>____________________________________<br>Hui Wang<br>School of physics, Fudan University, Shanghai, China<br>
</div>