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## Definition of total energy in Abinit and VASP

Moderators: jzwanzig, jolafc

kojikoji
Posts: 5
Joined: Wed Sep 30, 2020 8:46 am

### Definition of total energy in Abinit and VASP

Hello,

I am a personal Abinit user recently switched from VASP. I am confused with the definition of total energy in Abinit. ETOT is
about 2 order bigger than that of VASP, after accounting for the hartree/eV unit conversion.
Could someone please specify the difference of total energy between energy(sigma->0) in VASP and ETOT in Abinit?
Last edited by kojikoji on Thu Nov 05, 2020 8:54 am, edited 1 time in total.

ebousquet
Posts: 469
Joined: Tue Apr 19, 2011 11:13 am
Location: University of Liege, Belgium

### Re: Definition of total energy in Abinit and VASP

Dear kojikoji,
The total energy has no physical meaning, only energy differences have sense. Which one of the two are you comparing, if it is the total energy then each code can give a different one, depending on different settings, pseudos, etc.
Best wishes,
Eric

kojikoji
Posts: 5
Joined: Wed Sep 30, 2020 8:46 am

### Re: Definition of total energy in Abinit and VASP

Hello ebousquet,

Is there any way to get total energy, even if its meaningless? I would like to check if results are the same for both Abinit and VASP.
Also, I check the Abinit reference and found as below, so total energy itself is meaningless I agree:
the (pseudo) total energy (Etot) in Hartree [This is not the total energy of the system, since the pseudopotential approximation has been made: a constant energy (in the frozen-core approximation) should be added to the present pseudo total energy in order to obtain a total energy, that includes the contributions from the core electrons. Since only differences of total energy matter (except is extremely rare cases), one can work with this pseudo energy like if it were the true total energy, except that the missing constant depends on the pseudopotential that has been used. Thus one has to perform differences of pseudo energies between simulations that use the same pseudopotentials].
Perhaps you know the calculation procedure of the "constant energy" ? This may come in handy for calculating total energy. Thank you.

ebousquet
Posts: 469
Joined: Tue Apr 19, 2011 11:13 am
Location: University of Liege, Belgium

### Re: Definition of total energy in Abinit and VASP

Dear kojikoji,
I do not know this "constant energy" but it should be more or less easy to seek for it.
Do you belong to the "extremely rare cases" where the total energy matters?
Best wishes,
Eric