[Pw_forum] scan the lattice constant

Virginie Quequet virginie.quequet at polytechnique.fr
Fri Sep 9 09:14:57 CEST 2005

Dear Stefano

Are you sure, that, even for metal, the potential energy surfaces are
smooth when we use fixed number of PW's? Because some time ago, I have
problem of discontinuities with Ti and I try to impose fixed number of
PW's, but the discontinnuities did not disappeared. I have thought that
not good enough convergence in k-point was the reason, and that the
potential energy surface is perfectly smooth only in the limit of an
infinit number of k-point for metal. Is it wrong?

Virginie Quequet <virginie.quequet at polytechnique.fr>

From: Stefano Baroni <baroni at sissa.it>
Subject: Re: [Pw_forum] scan the lattice constant
Date: Thu, 8 Sep 2005 21:44:33 +0200
To: pw_forum at pwscf.org
Reply-To: pw_forum at pwscf.org

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Kostya: I beg to differ here. I do not want to open a thread "PW's  
vs. Gaussians" which would have the same chances to be settled as  
"coca vs. pepsi", but what  you say, while perfectly (kind of) clear  
to me, may be misleading to the less experienced.

PW potential energy surfaces (PES) are as smooth as they can be (i.e.  
*perfectly smooth*, or of class C(\infty), if you prefer) *with fixed  
boundary conditions* (i.e. for a fixed shape/size of the unit cell  
which determines the periodic boundary conditions). This smoothness  
has strictly nothing to do with any cancellation of errors. With a  
fixed number of PW's, PES's are perfectly smooth, but the  
(variational) accuracy would depend on the size/shape of the unit  
cell. Discontinuities come in when varying the boundary conditions  
because PW basis sets depend on boundary conditions (conceptually)  
for exactly the same reasons why localized basis sets depend on  
nuclear positions. With Gaussians you have "Pulay forces" (which  
require a special treatment going beyond Helmann Feynman), with PW's  
you have "Pulay stresses" (which require an equally special ad hoc  

It is indeed the ease with which accuracy can be assessed and  
improved with PW's which make the problem appear. As said, if one was  
ready to use a fixed basis set for different geometries (as one does  
with localized basis sets without really caring about the dependence  
of the accuracy on the geometry), the potential energy surface would  
be perfectly smooth, though less accurate.

As simple as that. No misterious powerful cancellation principles here.


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