# Re: [SIESTA-L] Total energy

```Thanks  for your response Prof. Artacho!

So if I have a situation like below for a large metallic system in which
FreeEng is lower than Etot, should I use Etot? Specifically, if I want to
plot the E-V curve from single point runs to obtain the equation of state I
should use Etot instead of FreeEng? Or if I want to compute the cohesive
energy, Etot should be used instead of FreeEng?```
```
siesta: Program's energy decomposition (eV):
siesta: Ebs     =      -859.157108
siesta: Eions   =      9568.777238
siesta: Ena     =       277.651800
siesta: Ekin    =      4247.415878
siesta: Enl     =     -1698.956164
siesta: Eso     =         0.000000
siesta: Edftu   =         0.000000
siesta: DEna    =       162.636813
siesta: DUscf   =        26.981770
siesta: DUext   =         0.000000
siesta: Exc     =     -6715.533109
siesta: eta*DQ  =         0.000000
siesta: Emeta   =         0.000000
siesta: Emolmec =         0.000000
siesta: Ekinion =         0.000000
siesta: Eharris =      -13268.580292
siesta: Etot    =        -13268.580250
siesta: FreeEng =    -13269.911499

On Tue, Jun 21, 2022 at 2:07 PM Emilio Artacho <e.arta...@nanogune.eu>
wrote:

>
> On 20 Jun 2022, at 17:16, Francisco Garcia <garcia.ff....@gmail.com>
> wrote:
>
> Dear users,
>
> In metallic systems with a fairly sizable electronic smearing temperature
> T, is it accurate to claim that
>
> (i) in a single point calculation, the free energy is the representative
> energy of the system (due to the addition of -TS to the total energy U)
>
>
> No, the -TS term for the phononic entropy is missing.
>
> It is the free energy defining the finite temperature equilibrium for the
> purely electronic
> problem for fixed external potential (fixed nuclei), as in Merminâ€™s finite
> -T DFT.
>
>
> (ii) in a variable cell optimization, the enthalpy is the representative
> energy of the system (due to the addition of the PV term to the energy).
>
>
> Yes, it is the free energy of the system (minimum defines equilibrium) for
> T=0.
>
> Emilio
>
>
> Thanks!
>
>
> --
> SIESTA is supported by the Spanish Research Agency (AEI) and by the
> European H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
>
>
> --
> Emilio Artacho
>
> Theory Group, Nanogune, 20018 San Sebastian, Spain, and
> Theory of Condensed Matter, Department of Physics,
> Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, UK
>
>
>
>
>
> --
> SIESTA is supported by the Spanish Research Agency (AEI) and by the
> European H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
>
```
```--
SIESTA is supported by the Spanish Research Agency (AEI) and by the European
H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
```