[Wien] Comparing Total Energies

[tran at theochem.tuwien.ac.at]

Hi,

When comparing two total energies obtained from different unit cells, one
has to be always very careful.

First, if you did the calculation on the
small unit cell with a k-mesh (n1,n2,n3), then the calculation on the
large unit cell should be done with the corresponding k-mesh
(n1/m1,n2/m2,n3/m3), where m1, m2, and m3 indicate by how many times
(along the lattice vectors a, b and c) is the large unit cell larger than
the small one. You can have problems if If n1/m1, n2/m2 or n3/m3 is not
an integer. But this not a problem if you
are using very good k-meshes such that the energies are well converged.

It can also happen that the orientation of the lattice vectors are
completely different in the small and large unit cells. In this case
some numerical noise (e.g., from the FFTs) can introduce some errors.

But actually, what do you mean by "I can't compare"?

My recommendation: use the same unit cell.  

F. Tran

On Tue, 10 Jul 2012, David Tompsett wrote:

> Dear All,
> 
> I have what is probably a very basic question about comparing total
> energies. I have been considering a system that undergoes a charge density
> wave transition from a high to low symmetry structure. I have been
> comparing the total energy of the high and low symmetry structures. The
> unit cell of the high symmetry structure has half as many atoms as does the
> low symmetry one.
> 
> In my calculations it seems I can't compare 2x(total energy of high
> symmetry cell) with 1x(total energy of low symmetry cell). Instead I have
> to perform the high symmetry calculation in the same size unit cell and
> symmetry as for the low symmetry. Why is this the case? What is the effect
> of having different sized unit cells?
> 
> Thank you,
> David Tompsett.
>