[Pw_forum] internal energy

[Jiayu Dai]

It is *NOT* any free energy of any physical system. It can be interpreted as the free energy of a system of electrons in the field of clamped nuclei, at the *fictitious* temperature corresponding to the smearing you use. The reason why this concept is useful is that the free energy is variational, while the internal energy is not. That's why Hellman-Feynman forces are derivatives of the free energy, but not of the internal energy.

Thanks Stefano. So, if we want to get the real physical free energy, we should do something else. As far as i know, it is difficult to estimate the free energy. There are several methods calculating it, one of which is from the phonon density of states. But i did not sure this method can be used at high temperature, because the anhormanic effect is important here.


Furthermore, how to get the internal energy here? The kinetic energy is easy to calculate, but how about the potential energy?



You do not need any internal energy. what you may actually want to estimate is the T->0 extrapolation of both the free and internal energies (which coincide in the T->0 limit). I think that some estimate of this are available in the pw output, but others may know more than me about this.

Yes, what you said is what i want. Maybe from the scf calculation, we can get something useful, cause scf gives out more information about energies. So, could some experts can said a little about this problem?

Thank all.

Jiayu


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Jiayu Dai
Department of Physics
National University of Defense Technology, 
Changsha, 410073, P R China
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