# Re: [SIESTA-L] Total energy

```Hello,
May I contribute my 0.01 ?. According to Eq.(21) in,```
```
Kresse & Furthmüller, Comp. Mat. Sci. 6 (1996) 15-50

a good estimate for the (T=0K) DFT energy is 1/2(Etot +FreeEng), and
this is the recipe I've always used.

Regards,
Roberto

On 22/06/2022 13:11, Francisco Garcia wrote:
```
```ThanksÂ  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 Â  Â  = Â  Â  Â -859.157108
siesta: Eions Â  = Â  Â  Â 9568.777238
siesta: Ena Â  Â  = Â  Â  Â  277.651800
siesta: Ekin Â  Â = Â  Â  Â 4247.415878
siesta: Enl Â  Â  = Â  Â  -1698.956164
siesta: Eso Â  Â  = Â  Â  Â  Â  0.000000
siesta: Edftu Â  = Â  Â  Â  Â  0.000000
siesta: DEna Â  Â = Â  Â  Â  162.636813
siesta: DUscf Â  = Â  Â  Â  Â 26.981770
siesta: DUext Â  = Â  Â  Â  Â  0.000000
siesta: Exc Â  Â  = Â  Â  -6715.533109
siesta: eta*DQ Â = Â  Â  Â  Â  0.000000
siesta: Emadel Â = Â  Â  Â  Â  0.000000
siesta: Emeta Â  = Â  Â  Â  Â  0.000000
siesta: Emolmec = Â  Â  Â  Â  0.000000
siesta: Ekinion = Â  Â  Â  Â  0.000000
siesta: Eharris = Â  Â Â  -13268.580292
siesta: Etot Â  Â = Â  Â  Â Â  -13268.580250
siesta: FreeEng = Â  Â -13269.911499

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

```
```    On 20 Jun 2022, at 17:16, Francisco Garcia <garcia.ff....@gmail.com
<mailto: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.Â

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!
Â

```
-- 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 SanÂ Sebastian, Spain, and
Theory of Condensed Matter,Â Department of Physics,
Cavendish Laboratory, University ofÂ 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/)

```
```
--
Dr. Roberto C. Pasianot         Phone: 54 11 4839 6709
Gcia. Materiales, CAC-CNEA      FAX  : 54 11 6772 7362
Avda. Gral. Paz 1499            pasia...@cnea.gov.ar
1650 San Martin, Buenos Aires
ARGENTINA
```
```--
SIESTA is supported by the Spanish Research Agency (AEI) and by the European
H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
```