<HTML><BODY style="word-wrap: break-word; -khtml-nbsp-mode: space; -khtml-line-break: after-white-space; ">Dear Paul: you are not doing anything wrong, but expecting something the code cannot do (not even if it was a Perfect Schrödinger Solver). Let me make a simple example that may convince you of what is missing. Take a monoatomic, periodic, linear chain out of equilibrium: what would the force acting on each atom be? 0, of course, by symmetry! How can it be, for a non-equilibrium configuration? Forces are zero by symmetry because you are considering (the code is considering) an infinite system. If the system was finite instead, forces on atoms near the surfaces would be nonvanishing, and the relaxation would (slowly) propagate from near the surface to deep into the bulk. The point is, in an infinite system vanishing forces are a necessary condition for equilibrium, but not a sufficient one. Vanishing forces AND stresses are both necessary and sufficient conditions for equilibrium to occur. If forces are already zero (by symmetry in your Al case, because each atomic site has cubic symmetry: notice that they are zero to machine precision, not just small) then you have to search for a configuration of zero pressure (which is large for your non-equilibrium configuration). This is achieved by vc-relax, or by fitting the E vs. volume relation to some equation of state, as suggested by somebody else in this forum recently. The latter is actually the preferred method for very simple systems (i.e. for systems whith very few degrees of freedom). Hope I did not misunderstood your question and that this clarifies. Yours - Stefano<DIV> <BR><DIV><DIV>On Apr 26, 2007, at 4:41 AM, Paul M. Grant wrote:</DIV><BR class="Apple-interchange-newline"><BLOCKQUOTE type="cite"><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><O:SMARTTAGTYPE namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="PlaceType"><O:SMARTTAGTYPE namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="PlaceName"><O:SMARTTAGTYPE namespaceuri="urn:schemas-microsoft-com:office:smarttags" name="place"><DIV class="Section1"><P class="MsoNormal"><FONT size="2" face="Arial"><SPAN style="font-size:10.0pt; font-family:Arial; font-size: 13.3333px; "><SPAN class="Apple-style-span" style="font-family: Arial; font-size: 13.3333px; ">To All: </SPAN><O:P style="font-family: Arial; font-size: 13.3333px; "></O:P></SPAN></FONT></P><P class="MsoNormal"><FONT size="2" face="Arial"><SPAN style="font-size:10.0pt; font-family:Arial; font-size: 13.3333px; "><SPAN class="Apple-style-span" style="font-family: Arial; font-size: 13.3333px; ">(BTW: PG: My last note on the “default” value of npk should have been termed “default maximum.” Sorry, but we Americans haven’t spoken English since 1776. Anyway, thanks for pointing me to the right Fortran routine.)</SPAN><O:P style="font-family: Arial; font-size: 13.3333px; "></O:P></SPAN></FONT></P><P class="MsoNormal"><FONT size="2" face="Arial"><SPAN style="font-size:10.0pt; font-family:Arial; font-size: 13.3333px; "><SPAN class="Apple-style-span" style="font-family: Arial; font-size: 13.3333px; ">I’m trying to do a very simple homework problem with PWscf…the optimization of the lattice constant of aluminum.</SPAN><O:P style="font-family: Arial; font-size: 13.3333px; "></O:P></SPAN></FONT></P><P class="MsoNormal"><FONT size="2" face="Arial"><SPAN style="font-size:10.0pt; font-family:Arial; font-size: 13.3333px; "><SPAN class="Apple-style-span" style="font-family: Arial; font-size: 13.3333px; ">Even with a wildly wrong first estimate of celldm(1) of 7.2 (the experimental value at RT is 7.665 au), I get zero net individual atomic forces and thus no optimization. I’ve gotten “relax” in PWscf to work with a lot more complex polymer monoclinic unit cell containing eight atoms and two different elements. So, I must be doing something really dumb (stupid)…maybe I should be using vc-relax instead? Anyway, here’s the I/O (scusi e grazie):</SPAN></SPAN></FONT></P></DIV></O:SMARTTAGTYPE></O:SMARTTAGTYPE></O:SMARTTAGTYPE></SPAN></BLOCKQUOTE></DIV><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; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><SPAN class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; 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-khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><FONT class="Apple-style-span" size="3"><SPAN class="Apple-style-span" style="font-size: 12px;"><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; ">---</SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></FONT></DIV><DIV style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><FONT class="Apple-style-span" size="3"><SPAN class="Apple-style-span" style="font-size: 12px;"><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; ">Stefano Baroni - SISSA</SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></SPAN></FONT><FONT class="Apple-style-span" size="3"><SPAN class="Apple-style-span" style="font-size: 12px;"><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; "><SPAN class="Apple-style-span" style="font-size: 12px; 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