I have had this doubt ever since I was introduced to DFT. For what I gathered whatever results you obtained with DFT (by using VASP, quantum expresso or any other software) you can say those properties are it, even for temperatures beyond 0K.
I don't understand why. For instance if DFT says it's ferromagnetic at 0K then why would you assume its ferromagnetic properties would stay sufficiently close to the ground state for T>0 as to state that.
I haven't read a theorem that says that excited states are close to the ground state. Does such theorem exist?
Edit: Quoting the book "A chemist's Guide to DFT" at the end of the section about the first Hohenberg-Kohn theorem **
One should note at this point that the ground state density uniquely determines the Hamilton operator, which characterizes all states of the system, ground and excited. Thus, all properties of all states are formally determined by the ground state density (even though we would need functionals other than ∫ρ(r)V dr + F [ρ], which is the functional constructed to deliver E0 but not properties of electronically excited states).
I don't follow it. Seems pretty basic cuz everyone just brushes over but I don't follow it. Why would the ground state characterize excited states?
** The first theorem basically says that the hamiltonian can be defined by the electronic density and can be made a unique functional of it.