Dear Fabien,<br><br>Thanks for the response. It is probably my k-mesh that is making it inaccurate to compare.<br><br>David.<br><br><div class="gmail_quote">On Tue, Jul 10, 2012 at 3:48 PM, <span dir="ltr"><<a href="mailto:tran@theochem.tuwien.ac.at" target="_blank">tran@theochem.tuwien.ac.at</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<br>
<br>
When comparing two total energies obtained from different unit cells, one<br>
has to be always very careful.<br>
<br>
First, if you did the calculation on the<br>
small unit cell with a k-mesh (n1,n2,n3), then the calculation on the<br>
large unit cell should be done with the corresponding k-mesh<br>
(n1/m1,n2/m2,n3/m3), where m1, m2, and m3 indicate by how many times<br>
(along the lattice vectors a, b and c) is the large unit cell larger than<br>
the small one. You can have problems if If n1/m1, n2/m2 or n3/m3 is not<br>
an integer. But this not a problem if you<br>
are using very good k-meshes such that the energies are well converged.<br>
<br>
It can also happen that the orientation of the lattice vectors are<br>
completely different in the small and large unit cells. In this case<br>
some numerical noise (e.g., from the FFTs) can introduce some errors.<br>
<br>
But actually, what do you mean by "I can't compare"?<br>
<br>
My recommendation: use the same unit cell.<br>
<br>
F. Tran<br>
<div><div class="h5"><br>
On Tue, 10 Jul 2012, David Tompsett wrote:<br>
<br>
> Dear All,<br>
><br>
> I have what is probably a very basic question about comparing total<br>
> energies. I have been considering a system that undergoes a charge density<br>
> wave transition from a high to low symmetry structure. I have been<br>
> comparing the total energy of the high and low symmetry structures. The<br>
> unit cell of the high symmetry structure has half as many atoms as does the<br>
> low symmetry one.<br>
><br>
> In my calculations it seems I can't compare 2x(total energy of high<br>
> symmetry cell) with 1x(total energy of low symmetry cell). Instead I have<br>
> to perform the high symmetry calculation in the same size unit cell and<br>
> symmetry as for the low symmetry. Why is this the case? What is the effect<br>
> of having different sized unit cells?<br>
><br>
> Thank you,<br>
> David Tompsett.<br>
><br>
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