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Dear Yuehua:<div><br><div><div>On Nov 12, 2007, at 4:00 AM, xu yuehua wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>hi everyone ,</div> <div>i am now do a relax job.my uint cell is a supercell . and i only want to relax a direction of lattice ,for example c .</div> <div>so i tried to vary the c .and compute. see the stress.but i do not know how to adjust the c on the base of the result like:<br clear="all"></div><p> entering subroutine stress ...</p><p> total stress (Ry/bohr**3) (kbar) P= -1.06<br> -0.00000893 -0.00000001 -0.00000001 -1.31 0.00 0.00<br> -0.00000001 -0.00000892 0.00000000 0.00 - 1.31 0.00<br> -0.00000001 0.00000000 -0.00000382 0.00 0.00 -0.56<br></p><div> <br class="khtml-block-placeholder"></div> <div>(10there is "-0.56" and the "-" stands for .i should the reduce c?</div></blockquote><div><br class="webkit-block-placeholder"></div>a positive pressure (or a positive component of the stress, for that matter) will make the material expand, if unconstrained.<br><blockquote type="cite"><div> <p> (2)0.56 is a large stres?</p></div></blockquote><div><br class="webkit-block-placeholder"></div>the answer is "NO", but this is an answer you could have answered yourself. assume you know the elastic constants of your material (if you don't, just take the experimental values, or any reasonable guess). how much would c vary given the known values of the elastic constants and of the zz componenty of the stress? unless the material is exceedingly soft, you will find that c will vary very little. you can practice this yourself. just let c vary by, say, 5%, and see by how much would \sigma_{zz} vary ...</div><div><br><blockquote type="cite"><div><p>(3) is there any other way to determine the suitable c?</p></div></blockquote><div>calculate and tabulate E vs. c and find the minimum. beware, though, that the curve that you will obtain should be rather rugged. this issue has been discussed several times in several places, including this forum, so I will stop here ...</div><blockquote type="cite"><div><p>thank you very much if you could give a advice .</p></div></blockquote><div><br class="webkit-block-placeholder"></div>you are most welcome</div><div>Stefano Baroni</div><div><br class="webkit-block-placeholder"></div><div> <span class="Apple-style-span" style="border-collapse: separate; 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