General question about total energy in ATK

[hadi9827]

Dear all,

I have a very basic question. If I optimize the same structure in two different situations (for example two molecules next to each other, first time with one specific orientation against each other and next time again the same molecules with a different orientation against each other), and I calculate the total energy in two cases, let's assume for the first case I get -50 eV and for the second one I get -55eV. Now I want to use these figures to say which orientation is more stable. How can I say that? I mean the sign of the total energy confuses me. I know that the one with less total energy is more stable, but with considering the negative sign or not? (-55 < -50 but |-55|>|-50|)?

Thanks

[filipr]

The total energy is basically the expectation value of the many-body DFT Hamiltonian. As such the sign matters: the lower the energy the more stable the configuration. Remember that adding an arbitrary constant scalar potential to the Hamiltonian does not change the physics (wave functions and density will be the same) but shifts the total energy. This mean that the actual value of the total energy is not of much use, only energy differences. In you example the important property is the difference in energy between two configurations ΔE = E_B - E_A. If ΔE is negative it means that configuration B is more stable than configuration A.

Note that the total energy is dependent on the pseudopotentials. So be sure that when you calculate energy differences between two configurations they should use the same pseudopotentials (you should in general try to use the same computational settings for the two calculations).

[hadi9827]

Thank you very much for the response.