[Wien] Comparing Total Energies

[David Tompsett]

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.
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