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<div class="moz-cite-prefix">On 12/5/14 5:11 AM, David Foster wrote:<br>
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cite="mid:1417774299.63759.YahooMailBasic@web125405.mail.ne1.yahoo.com"
type="cite">
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1- Cyrille, I fixed both in-plane and z-direction lattice constants. I need to fix the 15A vacuum in the z to prevent periodic interactions for studying adsorption.
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<br>
Dear David,<br>
<br>
Just my 2 cents to add to this.<br>
<br>
(a) Residual surface stress is a perfectly "legal" quantity. Some
people hunt for it (as well as for the surface elastic constants)<br>
specifically with DFT. You can check it out here:<br>
<br>
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<a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1103/PhysRevB.71.094104">http://dx.doi.org/10.1103/PhysRevB.71.094104</a><br>
<br>
or here<br>
<br>
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<a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1016/j.cma.2010.09.007">http://dx.doi.org/10.1016/j.cma.2010.09.007</a><br>
<br>
It is a lot of fun! :)<br>
<br>
<br>
(b) You have to be *extremely careful* when attaching something (a
molecule?) to your surface, as this will open a<br>
new can of worms. You will need to check if your combined system is
likely to have a dipole moment pointing perpendicularly to<br>
the slab -- and it might. Then you may have to keep an xy mirror
plane running through the middle of your slab [adding many more<br>
atoms to the calculation] or apply a dipole correction [makes
calculation convergence much slower]<br>
to get rid of it. If you don't, you run a chance of having a
spurious electric field in vacuum due to interaction between the<br>
polarized slab and is periodic images.<br>
<br>
There is an old paper that comments on the situation:<br>
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<a class="moz-txt-link-freetext" href="http://dx.doi.org/10.1103/PhysRevB.63.205426">http://dx.doi.org/10.1103/PhysRevB.63.205426</a><br>
<br>
but much more had been published since then.<br>
<br>
The really graceful way of studying slabs involves using a code that
can do 2D PBC in plane and break periodicity along the<br>
perpendicular direction. But then you have to abandon the PW basis
-- i.e., codes like VASP and QE -- and use something else.<br>
<br>
HTH,<br>
<br>
Serge<br>
<br>
<br>
<div class="moz-signature">
<div>-- <br>
Serge M. Nakhmanson <br>
Dep-t of Materials Science & Engineering, and Institute of
Materials Science <br>
University of Connecticut <br>
Storrs, CT 06269-3136 <br>
Phone: (860) 486-5252 <br>
<a href="http://satori.ims.uconn.edu/">http://satori.ims.uconn.edu/</a><br>
<br>
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