<div dir="ltr"><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000">Two alternatives:</div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000">a) For directions where you do not have inversion symmetry, the sum of the forces with a well converged calculation should be zero. You can use this to estimate from the force of the light atoms what those on the heavy atoms have to be.</div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000">b) Using clmextrapol (which is very efficient), calculate the forces by applying small displacements (finite differences).</div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000"><br></div><div class="gmail_default" style="font-family:verdana,sans-serif;color:#000000">A third option is to add a non-derivative optimizer to PORT, but this would require some real work.</div></div><br><div class="gmail_quote"><div dir="ltr">On Tue, Dec 4, 2018 at 9:41 AM Luc Fruchter <<a href="mailto:luc.fruchter@u-psud.fr">luc.fruchter@u-psud.fr</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">It has been said many times that forces are not correct when spin-orbit <br>
is included.<br>
Also, that the contribution of SO should generally be small, compared to <br>
energies that determine the equilibrium positions, so that minimization <br>
without SO should be a good approximation, for most cases.<br>
<br>
I wonder, however, if the following scheme may help to get confidence <br>
that a minimization without SO indeed provides a structure close to <br>
equilibrium:<br>
<br>
- perform minimization without SO,<br>
<br>
- switch-on SO on heavy atoms of the equilibrated structure,<br>
<br>
- fix these atoms, and relax the structure for the light atoms (like Oxygen)<br>
<br>
- check if this relaxed structure is close to the one obtained at the <br>
first step.<br>
<br>
<br>
Does this sound reasonable ?<br>
<br>
<br>
Thanks<br>
_______________________________________________<br>
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</blockquote></div><br clear="all"><div><br></div>-- <br><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><div dir="ltr"><div><div dir="ltr"><div><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><div dir="ltr"><span style="font-size:12.8px">Professor Laurence Marks</span><br></div><div dir="ltr"><span style="font-size:12.8px">"Research is to see what everybody else has seen, and to think what nobody else has thought", </span><span style="font-size:12.8px">Albert Szent-Gyorgi</span><br><a href="http://www.numis.northwestern.edu" target="_blank">www.numis.northwestern.edu</a> ; <span style="font-size:12.8px">Corrosion in 4D: </span><a href="http://MURI4D.numis.northwestern.edu" style="font-size:12.8px" target="_blank">MURI4D.numis.northwestern.edu</a><div><span style="font-size:12.8px">Partner of the CFW 100% program for gender equity, </span><a href="http://www.cfw.org/100-percent" style="font-size:12.8px" target="_blank">www.cfw.org/100-percent</a></div><div>Co-Editor, Acta Cryst A</div></div></div></div></div></div></div></div></div></div></div>