# Free energy based on BAR method

**URL:** <https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694>\
**Category:** User discussions\
**Created:** [August 21, 2020, 7:59pm UTC](https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694 "2020-08-21T19:59:32Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![dr\_m\_ganj](https://avatars.discourse-cdn.com/v4/letter/d/9f8e36/32.png) [@dr\_m\_ganj](https://gromacs.bioexcel.eu/u/dr_m_ganj)\
**Post date:** [August 21, 2020, 7:59pm UTC](https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694/1 "2020-08-21T19:59:32Z")

</div>

GROMACS version: 5.1.4  
GROMACS modification: Yes/No  
Dear Justin,  
1- I’ve read your tutorial entitle “Free Energy of Solvation”, you didn’t active the coulombic interactions between methane and water by setting “couple-lambda0 = vdw and coul\_lambdas = 0.00 0.00 0.00 …0.00”. I want to calculate the free energy of a functionalized Fullerene in water, it is true that I must use “couple-lambda0 = vdw-q” and coul\_lambdas = 0.00 0.05 …1.00"???  
2- How can I calculate the entropic contribution in this method?  
Best regards,  
Ganj

---

<div class="post-metadata">

**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [August 21, 2020, 11:56pm UTC](https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694/2 "2020-08-21T23:56:38Z")

</div>

My tutorial is explicitly for the vdW transformation only as it is a reproducible quantity from a well-known published article. If you want to compute a complete free energy of solvation, you need to transform the Coulomb terms as well by setting a series of λ states for them as well.

`gmx bar` prints ΔG and ΔS for neighboring windows.

---

<div class="post-metadata">

**Author:** ![dr\_m\_ganj](https://avatars.discourse-cdn.com/v4/letter/d/9f8e36/32.png) [@dr\_m\_ganj](https://gromacs.bioexcel.eu/u/dr_m_ganj)\
**Post date:** [August 22, 2020, 11:31am UTC](https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694/3 "2020-08-22T11:31:58Z")

</div>

Dear Justin,  
Many thanks, I’ll set “couple-lambda0 = vdw-q” and coul\_lambdas = 0.00 0.05 …1.00" in my .mdp file. But just a question about ΔS, how can I obtain ΔS? by subtracting the summation of S\_A from summation of S\_B?  
Best regrads,  
Ganj

---

<div class="post-metadata">

**Author:** ![jalemkul](https://dub1.discourse-cdn.com/flex017/user_avatar/gromacs.bioexcel.eu/jalemkul/32/18_2.png) [@jalemkul](https://gromacs.bioexcel.eu/u/jalemkul)\
**Post date:** [August 23, 2020, 12:35am UTC](https://gromacs.bioexcel.eu/t/free-energy-based-on-bar-method/694/4 "2020-08-23T00:35:31Z")

</div>

Presumably, but I’ve never cared about that quantity in any of the calculations I’ve done so I can’t say for sure.