Dear Fabien, Thanks for the response. It is probably my k-mesh that is making it inaccurate to compare.
David. On Tue, Jul 10, 2012 at 3:48 PM, <tran at theochem.tuwien.ac.at> wrote: > 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. > > > _______________________________________________ > Wien mailing list > Wien at zeus.theochem.tuwien.ac.at > http://zeus.theochem.tuwien.ac.at/mailman/listinfo/wien > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://zeus.theochem.tuwien.ac.at/pipermail/wien/attachments/20120711/e6ca7ddc/attachment-0001.htm>